How to Use Wi-Fi Calling Over Travel eSIM Cellular Data in 2026: Free Calls & SMS Abroad

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The IMS Architecture Explained: How Wi-Fi Calling Over Cellular Data Works

To understand how you can make zero-cost domestic calls and receive SMS two-factor authentication (2FA) codes while standing in Tokyo or Rome, you have to look beneath the operating system interface into the IP Multimedia Subsystem (IMS) architecture.

Modern mobile platforms—namely iOS (via "Backup Calling" / "Using Cellular Data") and Android (via "Cross-SIM Calling")—do not treat Wi-Fi Calling as a feature tied strictly to an 802.11 wireless radio. Instead, the OS treats Wi-Fi Calling as an untrusted non-3GPP IP access method.

When configured correctly, your primary SIM’s baseband firmware routes its entire voice and messaging stack directly through the secondary data pipe provided by your travel eSIM.

`` +-------------------------------------------------------------------+ | User Device (Dual SIM) | | | | [ Primary Home SIM ] [ Secondary Travel eSIM ] | (Voice/SMS Only) (MollySIM Data Pipe) | | │ │ | | ▼ │ | | IMS Voice / SMS (SIP & SRTP) │ | | │ │ | | ▼ │ | | IPsec Tunnel (IKEv2 / UDP 500, 4500) ──────────────┘ | +─────────────────────────────┬─────────────────────────────────────+ │ Encapsulated IPsec Packets ▼ [ Foreign Cellular Tower (Local IP) ] │ ▼ [ Public Internet ] │ ▼ [ Home Carrier ePDG Gateway (USA/UK/etc.) ] │ Decapsulated SIP/SRTP ▼ [ Home Carrier Core IMS Network / Billing ] ``

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1. The Protocol Stack: From Voice Codecs to IPsec Packets

When you initiate a voice call or receive an SMS over your primary number:

1. Signaling & Media Generation: The OS generates standard Session Initiation Protocol (SIP) signaling messages to initiate the session, while your voice is encoded into Secure Real-time Transport Protocol (SRTP) packets using adaptive codecs (such as AMR-WB or EVS).
2. IKEv2 Authentication: The device initiates an Internet Key Exchange version 2 (IKEv2) handshake aimed at your home carrier’s Evolved Packet Data Gateway (ePDG). Mutual authentication occurs transparently using your physical SIM or primary eSIM’s cryptographic credentials (USIM/ISIM).
3. Encapsulation via IPsec Tunnel: Once authenticated, an encrypted IPsec ESP (Encapsulating Security Payload) tunnel is established over UDP ports 500 and 4500.
4. Local Loopback Routing: Instead of directing these IPsec packets to an 802.11 Wi-Fi chip, the operating system routes them internally through the active packet data interface (pdp_ip0 on iOS) assigned to your secondary travel eSIM.

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2. Why Home Carrier Billing Engines See "Generic Wi-Fi"

The fundamental reason this strategy avoids daily roaming charges (such as AT&T’s $12/day International Day Pass or Verizon’s $10/day TravelPass) lies in how the home carrier’s core network processes ePDG traffic.

When your primary line connects via a foreign cell tower's roaming partner (e.g., Vodafone UK, Orange France, Docomo Japan), the visited network generates Transferred Account Procedures (TAP3) or Call Detail Records (CDRs). These records are sent back to your domestic carrier to bill interconnect fees.

In contrast, when voice packets are encapsulated inside an IPsec tunnel over your secondary data connection, the foreign tower only sees generic, encrypted data payload traversing its APN. The packets hit your home carrier’s ePDG directly over the public internet. Because the session originates from an untrusted non-3GPP IP endpoint, the carrier's policy and charging rules function (PCRF) rates the interaction as standard domestic Wi-Fi calling, instantly eliminating international surcharges.

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3. The Role of the Travel eSIM Transit Layer

For this architecture to maintain continuous, uninterrupted call fidelity, the secondary travel eSIM must deliver rock-solid IP packet delivery with low jitter and minimal packet loss. Dropped IPsec packets cause the IKEv2 session to renegotiate, leading to dropped calls and delayed SMS delivery.

This is where a high-performance travel data provider like MollySIM serves as an optimal transit layer:

• Direct Local Peering: MollySIM connects through low-latency local breakout points across global Tier-1 partner networks, reducing Round-Trip Time (RTT) to your domestic carrier's ePDG servers.
• Resilient FUP Architecture: Voice-over-IP (VoIP) streams demand minimal bandwidth (typically between 32 kbps and 64 kbps for AMR-WB/Opus streams), but they require consistent throughput. While conventional travel eSIM providers throttle post-allowance speeds down to an unusable 128 kbps, MollySIM maintains a 384 kbps Fair Use Policy (FUP) threshold—3x faster than the industry standard.

Even if you exhaust your high-speed allowance while navigating with Apple Maps or streaming media, MollySIM’s baseline bandwidth easily sustains the active IPsec tunnel, keeping your primary carrier's Wi-Fi Calling, 2FA SMS reception, and background authentication protocols fully operational.

Carrier Compatibility & Roaming Policy Matrix (AT&T, Verizon, T-Mobile, EE, Vodafone)

Not all mobile network operators (MNOs) treat cross-SIM Wi-Fi Calling equally. While the underlying 3GPP architectural framework for Evolved Packet Core (EPC) and IP Multimedia Subsystem (IMS) routing is standardized globally, individual carriers configure their remote ePDG (Evolved Packet Data Gateway) firewalls and entitlement configuration servers differently.

Understanding your home carrier's specific IMS provisioning rules, geo-filtering constraints, and roaming triggers is critical to prevent accidental roaming charges or IMS registration failures while abroad.

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Pre-Departure Prerequisites: The Domestic Handshake & E911 Registration

Before boarding an international flight, you must complete two non-negotiable provisioning steps while physically connected to your domestic carrier’s native cellular towers:

1. Cryptographic E911 Address Validation: US and European telecommunications mandates (including FCC and Ofcom regulations) require an active, validated physical emergency address stored on your carrier profile before IMS credentials can authenticate outside direct cellular reach. If you attempt to enable Wi-Fi Calling abroad without an active E911 address on file, the entitlement server will reject the profile installation.
2. Initial Domestic Handshake (IMS Token Generation): Toggle Wi-Fi Calling ON while connected to a local home Wi-Fi network and confirm that your status bar reflects carrier Wi-Fi Calling (e.g., T-Mobile Wi-Fi or VZW Wi-Fi). This exchanges the necessary IKEv2 security certificates and establishes the initial subscriber-edge authentication tokens. Performing this step for the first time while already overseas frequently fails because carrier provisioning servers reject cross-border bootstrap requests.

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Carrier Policy & IMS Provisioning Matrix (2026 Status)

The following matrix outlines current technical behaviors across tier-1 North American and European telecommunications operators:

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Operator-Specific Architectural Nuances

`` +-----------------------------------------------------------------------------------+ | CARRIER-SPECIFIC IMS BEHAVIOR | +-----------------------------------------------------------------------------------+ | | | [ T-Mobile US ] ---> Fully open ePDG ---> Automatic switch to Backup Calling | | | | [ Verizon ] ---> Prioritizes VZW Data ---> Requires Roaming Toggle = OFF | | | | [ AT&T ] ---> Aggressive MNO Lock ---> Requires Fake MNS Carrier Lock | | | +-----------------------------------------------------------------------------------+ ``

1. Verizon Wireless

Verizon natively supports "Wi-Fi Calling over Mobile Data". On dual-SIM Apple devices, Verizon’s IMS profile allows the baseband to route voice and SMS packets over an active travel data eSIM immediately upon losing native Verizon cellular signal. However, if your Verizon line detects even a marginal partner tower signal abroad, it will attempt to latch onto the local MNO, potentially triggering a $10–$12/day TravelPass session. To prevent this, data roaming must be permanently disabled on the Verizon profile before transit.

2. AT&T

AT&T implements aggressive roaming capture profiles. If your device sees an overseas partner network (such as Vodafone in the UK or Orange in France), AT&T’s SIM applet commands the modem to prioritize that roaming connection over secondary data routing, automatically triggering the $12/day International Day Pass (IDP).

• The Workaround: On iPhones, navigate to Settings > Cellular > AT&T > Network Selection, disable Automatic, and select an incompatible network (or a carrier with no roaming agreement with AT&T). This forces the AT&T baseband into a "No Service" state, which instantly commands iOS to initiate IMS registration over your secondary travel eSIM channel.

3. T-Mobile US & Google Pixel "Backup Calling"

T-Mobile provides the most transparent IMS handoff. On Android hardware (such as the Google Pixel 7/8/9/10 series), this protocol is explicitly integrated into Android OS under the name "Backup Calling". In the Pixel network settings, you can explicitly designate your MollySIM data line as the primary transport layer for T-Mobile voice and SMS calls, preventing carrier hunting entirely.

4. European & Canadian Carriers (EE, Vodafone, Telus)

While UK operators (like EE and Vodafone) largely permit global ePDG tunneling without geographic restrictions, Canadian carriers (Telus, Bell, Rogers) frequently employ firewall-level geoblocking on standard Wi-Fi calling. If your Canadian carrier rejects connections originating from non-Canadian IP addresses, having an eSIM with direct, low-latency transit routing is critical.

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Ensuring Network Isolation & Transit Reliability

To ensure that your domestic line routes exclusively through your secondary data line without incurring international roaming fees:

1. Disable Data Roaming on the Home Line: Leave the home line toggled ON for voice/SMS, but ensure Data Roaming is set to OFF.
2. Set Primary Data to Your Travel eSIM: Designate MollySIM as your dedicated cellular data connection with Cellular Data Switching turned OFF.

`` Settings > Cellular / Mobile Data ├── Primary SIM (Domestic): Voice/SMS ON | Data Roaming OFF | Network Selection MANUAL └── Secondary eSIM (MollySIM): Data ON | Roaming ON | Baseline 384kbps FUP Active ``

Because cross-SIM voice calls rely on background SIP keep-alives and continuous IPsec tunneling, network stability is essential. If your data connection is throttled down to legacy 128 kbps limits, a sudden burst of background data (like an incoming email or navigation recalculation in Google Maps) can starve the voice stream, causing immediate call drops.

With MollySIM’s 384 kbps Fair Use Policy (FUP) baseline speed—3x faster than the 128 kbps industry standard—your secondary line preserves enough dedicated throughput to keep real-time voice packets, Apple Pay token authentications, and mapping navigation fully operational simultaneously, even after you have exhausted your primary high-speed data tier.

Step-by-Step iOS Walkthrough: Activating 'Carrier Using Cellular Data' on iPhone

Configuring cross-SIM Wi-Fi Calling (internally routed as Apple's Backup Calling or IMS over alternate data interface) requires strict separation of your voice and data pathways. Follow this exact click-by-click walkthrough for iPhones running iOS 17, iOS 18, and newer versions to ensure your domestic line operates 100% free over your travel data.

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Step 1: Assign Cellular Data to Your Travel eSIM

Your iPhone must explicitly pull all internet packets through the travel eSIM rather than your domestic carrier.

1. Open Settings > tap Cellular (or Mobile Data depending on your region).
2. Tap Cellular Data.
3. Select your travel eSIM profile (e.g., MollySIM).
4. Return to the main Cellular menu, tap Default Voice Line, and confirm it remains assigned to your Primary (Domestic) carrier. This preserves your standard iMessage phone number mapping and standard outgoing caller ID.

`` Settings > Cellular ├── Cellular Data ─────────► [MollySIM] └── Default Voice Line ────► [Primary / Home Carrier] ``

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Step 2: Disable 'Allow Cellular Data Switching' (Zero Leakage Rule)

This is the most critical safeguard to avoid inadvertent, astronomical per-megabyte international roaming charges.

1. Under Settings > Cellular > Cellular Data, locate the Allow Cellular Data Switching toggle.
2. Toggle this setting strictly to OFF.

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Step 3: Enable Native Wi-Fi Calling on Your Primary Line

Before your domestic carrier can route over a cellular data pipe, its Wi-Fi Calling protocol must be enabled.

1. Navigate to Settings > Cellular.
2. Under the SIMs section, tap your Primary line.
3. Tap Wi-Fi Calling and toggle Wi-Fi Calling on This iPhone to ON.
4. Confirm any carrier E911 emergency address prompts if required by your domestic operator (such as AT&T, T-Mobile, or Verizon).

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Step 4: The Network Selection Override Trick

If your domestic carrier has international roaming agreements in your destination country, iOS will default to standard cellular roaming—exposing you to voice and SMS roaming fees. To force iOS into routing calls over your eSIM's data stream, you must deliberately sever the home SIM's direct tower connection.

1. In Settings > Cellular, select your Primary line.
2. Tap Network Selection.
3. Toggle Automatic to OFF.
4. Wait 15–30 seconds while iOS scans local cell towers, then select an incompatible or non-partner carrier from the list (or any network that rejects your home SIM).
5. Your domestic line will lose direct tower access and briefly show "No Service".

Once iOS registers that your domestic SIM has "No Service" while your MollySIM data line has an active internet connection, Apple's CoreTelephony stack automatically triggers a direct IPsec tunnel over the eSIM data pipeline.

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Step 5: Visual Confirmation via iOS Control Center

To confirm that your setup is working correctly without incurring roaming fees:

1. Swipe down from the top-right corner of your screen to open the iOS Control Center.
2. Look at the carrier status header in the top-left corner.
3. You should see:

``text [Primary Carrier] using Cellular Data [MollySIM / Local Network Name] ` (On older iOS builds, this may render as [Home Carrier] over Cellular Data or [Home Carrier] via [eSIM]`).

Once this indicator appears, every domestic call you make, every SMS 2FA code you receive, and every incoming voice call from back home will route seamlessly through your travel data pipe. Thanks to MollySIM’s 384 kbps Fair Use Policy (FUP) baseline, this connection maintains clean SIP voice packet delivery with low jitter alongside your background apps, Apple Pay verifications, and mapping navigation.

Android Configuration: Enabling 'Backup Calling' on Pixel and Samsung Galaxy Devices

While Apple unifies cross-SIM Wi-Fi calling under an automated, hidden IPsec mechanism, the Android ecosystem exposes these features directly through OEM-specific implementations called Backup Calling or Auto Data Switching. In modern Android architectures, the operating system uses Android's native IMS (IP Multimedia Subsystem) framework to route your physical primary SIM’s voice and SMS traffic over the secondary eSIM’s IP data pipe.

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Step 1: OEM-Specific Navigation Paths

Depending on your device manufacturer, the exact settings toggle and naming conventions vary:

`` ┌─────────────────────────┬───────────────────────────────────────────────────────────────────────────┐ │ Device Platform │ Configuration Path & Toggles │ ├─────────────────────────┼───────────────────────────────────────────────────────────────────────────┤ │ Google Pixel (Pixel UI) │ Settings > Network & internet > SIMs > [Primary SIM] > Toggle │ │ │ "Backup Calling" to ON. │ │ │ Ensure "Mobile data" is set to your MollySIM profile. │ ├─────────────────────────┼───────────────────────────────────────────────────────────────────────────┤ │ Samsung Galaxy (One UI) │ Settings > Connections > SIM manager > Toggle "Auto data switching" to ON.│ │ │ Under "Preferred SIMs", set "Mobile data" to MollySIM. │ │ │ Open Settings > Connections > Wi-Fi Calling > Enable on Primary SIM. │ ├─────────────────────────┼───────────────────────────────────────────────────────────────────────────┤ │ OnePlus (OxygenOS) │ Settings > Mobile network > [Primary SIM] > Enable "Wi-Fi Calling". │ │ │ Return to Mobile network > Enable "Use mobile data during calls". │ └─────────────────────────┴───────────────────────────────────────────────────────────────────────────┘ ``

Manual Network Lock: Just like on iOS, navigate to Settings > Network & internet / Connections > Mobile networks > Network operators on your domestic SIM, turn off "Select automatically", and lock onto an unavailable network to force the line into searching mode.

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Step 2: Verifying IMS Registration Status via Testing Dialer Code

To verify with 100% certainty that your domestic line is successfully tunneling its voice and SMS interfaces over your MollySIM data profile, inspect Android's low-level telephony stack:

1. Open your default Phone dialer app.
2. Dial ##4636## to launch the Android Testing Menu. (Note: On some Samsung builds, you may need to use Samsung’s ServiceMode via #0011# or install an activity launcher).*
3. Tap Phone information.
4. Select Phone 0 (typically your physical primary SIM slot) from the top dropdown.
5. Tap the three-dot overflow menu in the top-right corner and select IMS Service Status.
6. Verify the following values:

• IMS Registration: Registered
• Voice over Wi-Fi: Available
• SMS over IP / IMS: Available

If Voice over Wi-Fi reads Unavailable, toggle Airplane Mode on and off while keeping your secondary MollySIM mobile data active to trigger re-registration with your home carrier’s ePDG (Evolved Packet Data Gateway).

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Step 3: DSDS Resource Management & Battery Optimization Settings

Android devices operate on Dual SIM Dual Standby (DSDS) architecture, meaning both lines share a single transceiver. If Android’s memory manager or battery optimizer terminates background telephony services, cross-SIM IMS packets will drop.

1. Prevent Background Telephony Sleep

Aggressive manufacturer task killers (especially on Samsung One UI and Xiaomi MIUI/HyperOS) can kill the IMS daemon when screen-off states persist.

• Go to Settings > Apps > Special app access > Battery optimization.
• Select All apps, locate SIM Toolkit, Phone Services, and Carrier Services, then set them to Unrestricted / Don't optimize.

2. Bandwidth Contention and Low Jitter Assurance

Cross-SIM SIP signaling requires steady, non-zero throughput. Competitor travel eSIMs that throttle to a standard 128 kbps FUP will cause the Android IMS client to experience continuous packet loss, tearing down the virtual IPsec tunnel and resulting in missed calls.

Because MollySIM’s Fair Use Policy provides a 384 kbps baseline—three times the industry standard—your Android device maintains unfragmented voice codecs (AMR-WB) and rapid SMS 2FA delivery in the background, even while actively running Google Maps navigation or handling Google Wallet tokenization.

Cost & Performance Benchmark: Carrier Roaming vs. Pocket Wi-Fi vs. MollySIM Backup Calling

Navigating international connectivity requires balancing cost, technical reliability, and operational footprint. While traditional carrier travel passes offer zero-configuration voice access, their pricing models remain predatory. Conversely, standard data-only eSIMs strip away your primary identity, breaking critical banking and verification workflows.

The benchmark below evaluates the four dominant international connectivity strategies over a standard 14-day international deployment:

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The Engineering Reality: Why Cross-SIM IMS Tunnels Collapse on Generic eSIMs

Wi-Fi Calling over Cellular Data does not treat voice packets like regular web traffic. When your phone bridges your home line across an active MollySIM data stream, it constructs an IPsec ESP (Encapsulating Security Payload) tunnel over UDP port 4500 directly back to your home carrier’s Evolved Packet Data Gateway (ePDG).

`` [ Primary Carrier Voice Engine ] │ ▼ [ IKEv2 / IPsec Encapsulation (UDP 4500) ] │ ▼ [ MollySIM Local Breakout Core Engine (Low Latency) ] │ ▼ [ Home Carrier ePDG Core ] ───► [ Clear Voice (AMR-WB / EVS Codec) ] ``

This real-time transport architecture introduces strict telecommunication constraints:

1. Codec Starvation & Jitter Buffering (EVS vs. AMR-WB)

Modern high-definition voice uses Enhanced Voice Services (EVS) and Adaptive Multi-Rate Wideband (AMR-WB) codecs. These codecs operate at bitrates between 12.65 kbps and 24.4 kbps, transmitting Real-time Transport Protocol (RTP) packets every 20 milliseconds.

When a standard budget travel eSIM throttles down to the industry-standard 128 kbps Fair Use Policy (FUP), packet serialization delay spikes. A single background operating system process—such as an Apple Pay token refresh or background email fetch—saturates the entire 128 kbps pipe. The resulting jitter (>100ms) and packet loss (>3%) cause the smartphone’s jitter buffer to overflow, producing robotic voice artifacts, one-way audio, or abrupt RTP stream teardowns.

2. SIP Session Termination and 384 kbps Headroom

SIP signaling (RFC 3261) relies on continuous keep-alive polling (SIP OPTIONS / NOTIFY frames) over the IPsec tunnel. If latency spikes beyond 2000ms due to data bandwidth starvation, your home carrier’s IMS core presumes the client has disconnected and sends an ungraceful SIP 408 Request Timeout, deregistering the primary phone line.

MollySIM solves this through an architectural 384 kbps FUP baseline:

• Triple the Bandwidth: At 384 kbps, there is sufficient headroom to sustain the 30 kbps bidirectional IPsec RTP audio stream alongside low-overhead background operations (like Google Maps vector tile pre-fetching and essential push notifications).
• Local Breakout (LBO) Routing: Rather than backhauling your traffic halfway around the world to a distant proxy server, MollySIM routes packets through localized tier-1 edge nodes. This slashes packet Round Trip Time (RTT) to the target ePDG, eliminating the latency spikes that trigger emergency SIP disconnects and dropped 2FA verification texts.

Troubleshooting, 2FA SMS Delivery, and Avoiding Hidden Billing Traps

Operating Wi-Fi Calling over secondary eSIM cellular data is a reliable strategy for international travel, but running real-time voice and SMS through an untrusted IPsec tunnel over a foreign cellular data layer introduces specific edge cases. Below is how to diagnose and resolve failure modes, protect against carrier roaming fees, and ensure incoming bank authentication texts arrive without interruption.

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1. Diagnosing Failed 2FA Shortcodes and Bank SMS Delivery

Authentication texts (A2P SMS) rely on SMS over IMS (SIP MESSAGE payloads) when operating over Wi-Fi Calling. If standard peer-to-peer texts arrive but six-digit banking codes fail, the issue typically stems from one of two failure points:

• SIP Registration Drop During Inactivity: If your travel data connection throttles below the threshold needed to maintain SIP keep-alive handshakes, your home carrier’s core silently deregisters your line from the ePDG. While standard data apps appear connected, incoming SMS cannot route. Using a high-throughput travel profile like MollySIM—which maintains an unthrottled baseline and an industry-leading 384 kbps Fair Use Policy (FUP) (3x the standard 128 kbps limit)—ensures background services like Google Maps vector fetches or Apple Pay token syncs do not starve the IPsec signaling channel.
• Carrier Shortcode Interconnect Routing: Some financial institutions explicitly restrict automated OTP shortcodes from dispatching across unverified non-domestic IP addresses. If shortcodes do not arrive, toggle Airplane Mode ON for 10 seconds and OFF to force an immediate re-keying of the IKEv2/IPsec security association with your domestic carrier's IMS gateway.

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2. Eliminating Hidden Roaming Charges: The "Core Switch Hard-Lock"

The single biggest financial trap when using Dual SIM abroad is the automatic trigger of daily roaming packages (such as AT&T’s International Day Pass or Verizon’s TravelPass).

Simply toggling "Data Roaming" to OFF on your primary physical SIM/eSIM is not sufficient. If your primary line momentarily registers with a foreign cell tower to search for 2G/3G/LTE signaling:

1. An incoming spam call or background SMS delivery attempt pings the foreign visited network (VLR).
2. The foreign tower acknowledges the handshake.
3. Your domestic carrier logs international presence and automatically bills you the $10–$15 daily pass fee—even if you never picked up the call.

`` +-----------------------------------------------------------------------------------+ | THE DUAL-SIM ROAMING TRAP | | | | [ Foreign Cell Tower ] <--- (Handshake) ---> [ Primary Home SIM ] | | | | | | v v | | Billed $10-$15 Daily Fee! "Data Roaming: OFF" | | (Does NOT stop voice/SMS ping!) | +-----------------------------------------------------------------------------------+ ``

How to Hard-Block Roaming at the HLR/HSS Switch Level

To completely insulate your account:

1. Contact Your Carrier Prior to Departure: Request an "International Roaming Block" (IRB) or "Zero-Dollar Roaming Cap" added directly to your line provisioned inside their Home Location Register (HLR) / Home Subscriber Server (HSS).
2. The Result: The home carrier will reject all roaming connection agreements at the core network level. Your primary line will register zero signal bars from local foreign towers and will be forced to route voice and SMS exclusively via the Wi-Fi Calling protocol over your MollySIM data connection.
3. Alternative (Manual Network Selection): If your carrier does not support an IRB feature code, go to Settings > Cellular > Network Selection on your primary line, disable "Automatic", and manually select an incompatible local carrier. This locks the primary SIM into an unattached state, forcing it into "Using Cellular Data" backup mode.

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3. Resolving One-Way Audio and Geo-Blocked IMS Gateways

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Pre-Flight to Touchdown: The Ultimate Activation Checklist

Follow this exact sequential workflow to ensure zero billing surprises and instant connectivity upon arrival:

`` [ Departure Gate ] ──> [ Mid-Flight ] ──> [ Destination Touchdown ] ──> [ Final Verification ] ``

Step 1: At the Departure Gate (Home Country)

• Verify Wi-Fi Calling is enabled and tested on your primary line while connected to airport Wi-Fi.
• Install your travel eSIM profile (e.g., via the MollySIM app).
• Label your SIM cards clearly in device settings: label your domestic line as "Primary" and the travel profile as "Travel Data".
• Ensure an International Roaming Block is active on your primary carrier line.

Step 2: In Flight (Cruise Altitude)

• Turn Airplane Mode ON.
• Set Cellular Data to default to your Travel Data eSIM.
• Turn Data Roaming: ON for the Travel Data eSIM.
• Turn Data Roaming: OFF for your Primary line.
• Ensure "Allow Cellular Data Switching" is toggled OFF to prevent OS-level fallback leaks.

Step 3: Destination Touchdown (Runway)

• Turn Airplane Mode OFF.
• Wait 30–60 seconds for the travel eSIM to attach to the local partner tier-1 network.
• Look at your status bar / Control Center: Your primary SIM status must read [Carrier] using Cellular Data (iOS) or Backup Calling (Android).

Step 4: Final Verification

• Trigger a test SMS or dial an automated domestic number (e.g., balance check or voicemail).
• Verify zero roaming alerts or daily pass activation text messages arrive from your home provider.