My calls all fail

What phone do you have? You are probably using a phone that ATT does not support for VoLTE, which is required for calls. 

I have an iPhone 12, what is VoLTE?

Voice over LTE but an iPhone 12 should work fine but you do need to be connected to LTE or 5G. If you are seeing 4G on your display, that is actually ATT’s 3G network which they are in the process of shutting down. Check your phone settings, make sure 5G and LTE are enabled. Unfortunately, you need to contact your provider for assistance. Many international roamers are having issues due to the 3G network shutdown. 

ok, but even when calling over wifi I'm still getting the same problem. I've enabled 5G and LTE but still connected to 4G. 

Hi smxrphy, we are here to help you make/receive calls again on our AT&T Network on your iPhone 12 device when roaming!

Here are some few troubleshooting steps that are known to help troubleshoot wireless calls after the 3G shutdown, AT&T has phased from the 3G Network to make room for a newer and even better network:

1. Reset your network settings to refresh your connection, go to Settings>General management>Reset>Reset network settings. We also recommend toggling off/on your Airplane Mode to reset your connection to our nearest tower. 
2. Ensure your device software is updated to the latest software version, this ensures your device and all your apps are running smoothly. Navigate to check for software updates to check for any available software updates.
3. Add Access Point Names(APN) to your non AT&T device, this programs your data settings to enable you to use your data and is known to help fix calls concerns after the 3G sunset. 
4. Lastly ensure that your roaming is enabled on your device by navigating to the Settings App>Cellular>Cellular data options>Data roaming switch>on, to connect your device to our wireless network signal when available.

Feel free to reach back out to us on AT&T Community Forums to let us know if this worked out for you! Thank you for reaching out to the AT&T Community Forums!

Rhoda, AT&T Community Specialist 

Likely your foreign sim isn't able to work on at&t HD voice, even though the phone is capable.