Choosing the default voice and data line on iOS

When you install a second line on iPhone, iOS asks you to pick a default voice line (which line is used when you start a call) and which line uses cellular data. For Eflexsim travel use, keep your home line as the default voice line. It has your callable phone number. And set Eflexsim as the data line so apps and browsing use the cheaper travel plan. The default SMS line and per-contact line overrides are smaller adjustments you can make later.

Below: what each setting controls, the right choice for travel, and how to change them after install.

What each setting actually controls

iPhone separates dual-SIM behavior into a few independent settings:

Default Voice Line. When you tap a number in Contacts or dial from the keypad, iPhone uses this line. The setting doesn't affect calls you make through internet apps (WhatsApp, FaceTime, Signal). Those bypass the cellular voice infrastructure entirely.

Cellular Data. When apps need internet (Safari, Maps, Instagram, email), iPhone routes data through whichever line is selected here. Each line uses its own cellular plan independently.

iMessage & FaceTime. A separate setting under Settings → Messages and Settings → FaceTime. You pick which lines can send/receive iMessages and FaceTime. Most people use their home Apple ID and home number for this; the eSIM doesn't usually need its own iMessage activation.

Per-contact preference. Inside any contact, you can set which line auto-routes calls to that person. Useful if you have specific contacts you always reach on a specific number (rare for most travelers).

The right setup for travel

For 95% of travelers, the recommended setup is:

• Default Voice Line: your home line. Your home number stays the one you call from when you tap a number.
• Cellular Data: Eflexsim. Cheap travel data, your apps automatically use it.
• iMessage & FaceTime: keep on your home number. iMessage threads stay attached to your home identity; group chats with friends and family don't change because of a travel eSIM.
• Per-contact preference: leave at "Default" for everyone. Manually override only if you have a specific use case.

The reason this combination works: Eflexsim is data-only. It doesn't have a callable phone number. Setting it as your default voice line wouldn't make sense; you can't dial through a number that doesn't exist. Your home line keeps doing what it does (calls and SMS), and Eflexsim takes over the data your home plan would charge roaming fees for.

How to set defaults during install

iPhone walks you through these settings the first time you install a second line. The flow:

1. After the eSIM profile downloads, iOS asks you to label both lines. Pick names like "Personal" for your home line and "Eflexsim Italy" for the travel line.
2. iOS asks "Set up your Default Voice Line". Pick your home line.
3. iOS asks "Choose Which Line To Use For Cellular Data". Pick Eflexsim for travel use, or your home line if you're not abroad yet (you can switch later).
4. iOS asks "Set Up iMessage & FaceTime". Pick your home line. You can use both lines for iMessage, but most people stick with their home identity.

You can change any of these later in Settings without reinstalling the eSIM.

How to change them after install

If you skipped a prompt or want to change something later:

Default Voice Line:

1. Settings → Cellular.
2. Tap Default Voice Line.
3. Pick your home line.

Cellular Data:

1. Settings → Cellular.
2. Tap Cellular Data.
3. Pick the line for data. Eflexsim while abroad, your home line when home.

iMessage & FaceTime:

1. Settings → Messages → iMessage.
2. Tap Send & Receive. The addresses (your phone number and Apple ID) are listed. You can pick which line iMessage uses for outgoing messages.

Per-contact preference:

1. Contacts → tap the contact → Edit.
2. Scroll to Preferred Cellular Plan.
3. Pick the line for calls to that contact. Save.

Why iPhone asks you to set Default Voice Line during install

iPhone's prompt is specifically about voice calls because voice routing has more options than data. A typical user setup:

• One personal line, one work line: pick the work line for default voice so work calls outgoing.
• Two personal lines (home + travel eSIM): pick the home line because the travel eSIM is data-only.
• Two business lines: pick whichever is your "primary" line for the work you do most often.

For travel eSIM specifically, the prompt is somewhat unnecessary because Eflexsim doesn't offer voice. You essentially have to pick your home line. iOS still asks the same question regardless, because the same prompt covers all dual-SIM scenarios.

Smart Network Switching (a sometimes-helpful Android feature)

Pixel and Samsung have an automatic version of this that iPhone doesn't: if your default data line drops signal, the phone can auto-switch the data to the other line temporarily. For travel, this is double-edged:

• Helpful: if Eflexsim has a brief outage, your phone falls back to your home line so you stay online. Then it switches back.
• Risky: if your home line connects to roaming briefly during the fallback, you might get charged. Some travelers have been billed $5-20 for unintended home-line roaming this way.

iPhone doesn't have automatic data-line switching, which is actually a feature for travel. You stay explicitly on Eflexsim until you change it. If you're on Android and concerned about accidental roaming, turn the auto-switch feature off in Settings → Network → SIM management.

For the Android equivalent of these settings, see Pixel install and Samsung install. For more on switching lines day-to-day, see how to switch between your physical SIM and eSIM.