As far as I can tell, I'm getting the exact same issue as reported here. My text and data work perfectly fine, but I cannot send nor receive calls. Incoming calls just get my voicemail. Outgoing calls sit on the dialing screen, and eventually time out. I started noticing this behavior on 9/21, and I know calling was working as of 9/19.
While trying to dial, the 4G/5G status indicator go away, and then eventually I lose all signal. As soon as the call is ended, all signal immediately returns. Wifi calling does not work either.
I spent an hour on the phone with T-mobile, and as far as they could tell everything looked good from their end.
VoLTE is on, 5G is on, I've power cycled the phone multiple times, and tried a bunch of different call settings. The fix from the thread above does not work, as I don't have an option to enable 2G, though it's set to use 2G/3G/4G/5G.
After trying everything short of a factory reset/firmware downgrade, I finally seem to have fixed it. In the end it seems like I somehow have a (partially) dead SIM slot. I swapped which slot my SIM card is in, and viola, it just works like normal. Very strange. As soon as I popped it in the VoLTE indicator popped up, which I haven't seen in weeks, and I was able to make calls.