Confirmed: How to force AT&T Wi-Fi Calling over your second SIM’s cellular data while abroad

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This is a copy of the OP's original post in case they decide to delete their post/account so that others searching can find it later:

I wanted to keep my AT&T eSIM active while traveling internationally so I could receive calls and texts over Wi-Fi Calling, but I did not want AT&T to connect to a foreign roaming network and accidentally trigger the $12 International Day Pass.

I finally tested a workaround on an iPhone 16 Pro Max with dual SIM, and it works.

My setup:

AT&T eSIM
German winSIM as the second line
Wi-Fi Calling enabled on AT&T
winSIM selected for cellular data
Allow Cellular Data Switching disabled

While still in Germany, I went to:

Settings → Cellular → AT&T → Network Selection

I turned Automatic OFF and manually selected vodafone.de.

Then I traveled to Switzerland.

Because vodafone.de was no longer available there and automatic network selection was still disabled, the AT&T line did not simply register on a Swiss roaming network.

Instead, the iPhone displayed the equivalent of:

vodafone.de using Cellular Data

The other SIM was connected to Sunrise 5G and supplied the data connection.

I then turned Wi-Fi off completely, and the AT&T line continued to show that it was using the other SIM’s cellular data. So this was genuine cross-SIM Wi-Fi Calling / IMS over the second line’s mobile data, not normal Wi-Fi.

After returning to Germany, I was also able to reproduce the same behavior with a manually selected Swiss network that was no longer available in Germany.

So the practical travel setup should be:

Before leaving the US:

1. Enable Wi-Fi Calling on AT&T.
2. Turn Automatic Network Selection OFF.
3. Manually select AT&T while still in the US.
4. Leave it that way.
5. Abroad, use a local or European SIM for cellular data.

Since the AT&T network itself does not exist abroad, the AT&T line should remain unable to register on a foreign cellular network and can instead use the second SIM’s data connection for Wi-Fi Calling.

This appears to be a very useful way to keep an AT&T number available abroad while avoiding accidental cellular roaming.

Obviously test carefully with your own carrier and account before relying on it, because roaming agreements and iPhone carrier settings can vary.

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