No sound from my mic in phone calls

My fairphone 5 worked fine in Ireland. I moved back to the US, and got a new sim from T Mobile. Now, when I make a call, the person I am calling cannot hear me. I can still use video calling apps and record videos with audio, but during actual phone calls, I can hear the other person and they cannot hear me.

There was some setting I’d been turning on and off in Android’s Settings app that temporarily made it work again, but I can’t remember now what setting that was and it’s driving me up the wall. I feel like it was turning off the sim and turning it back on without physically removing it, or something like that, but I don’t see any setting that looks like what I did last time. I realize this is unhelpful, it’s driving me nuts, too. It also didn’t always work.

The phone had the no audio problem when I first switched to T mobile, intermittently and could sometimes but not always be fixed by reboots and/or toggling that setting. Then it worked completely fine for 3 weeks. Now it is broken again, for the past 3 days when I call somebody they cannot hear me.

I reached out to T Mobile support the first time it wasn’t working, and they didn’t have any suggestions.

OS: Android 14, no updates available
Problem occurs: with a T Mobile sim card (have not tried other American carriers), intermittently but continues for a long time once it starts
Tried to resolve: Turned phone off, put sim card out and back in again, turned phone back on. Tried making calls with and without wifi calling on, neither works.

I just tried switching the Access Point Name to the other of the two options, and that did not work.

Did you try to enable/disable loadspeaker shortly?