iPhone Calls Going Out On The Wrong Line: How to Fix It

Dual-SIM calls and data using the number you did not intend. This page explains what it does, how to use it on an iPhone and which carriers it applies to.

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Why does this happen and how do I fix it?

A dual-SIM iPhone makes three separate choices, and confusing them is the root of nearly every complaint here. Default Voice Line decides which number outgoing calls use by default. Cellular Data decides which line carries data. And each contact carries its own remembered line, which silently overrides the default for that person.

The contact-level memory is what produces the mystery, and Apple states the rule plainly: by default, your iPhone uses the same number that you used the last time you called that contact. Call somebody once from the second line and every later call to them goes out on that line, no matter what the global default says. You change it by opening the contact and tapping the word default above their number.

There is also a hardware limit worth knowing. Apple documents that the iPhone uses one cellular data network at a time, so while you are on a call on one line, the other line shows No Service — and if the call is not on your default data line, you need Allow Cellular Data Switching turned on to keep data flowing.

What should I try, in order?

1. Open Settings, Cellular, Default Voice Line and set the number you want outgoing calls to use.
2. Open the contact whose calls go out wrong, tap the word default shown with their number, then tap the number you want to use for them from now on.
3. Label your lines clearly under Settings, Cellular, each line, Cellular Plan Label. The line picker at the top of the keypad becomes far easier to read.
4. Set Settings, Cellular, Cellular Data to the line whose allowance you want to use, and turn on Allow Cellular Data Switching if you want data to continue during a call on the other line.
5. Check Settings, Apps, Messages, Send & Receive so texts leave from the number you expect too — it is set separately from calls.
6. Before each call, glance at the line indicator at the top of the keypad. It shows which number the call will go out on and it is the fastest confirmation.

Which carriers does this apply to?

Published for	any iPhone, Verizon, AT&T, T-Mobile and UScellular
Type	iOS setting
Good to know	Applies to any iPhone running two lines. Data switching behaviour depends on both carriers supporting it.

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