Call Failed / Can’t make or receive phone calls

I’m posting this thread on behalf of my sister. The device in question is a iPhone 7 from T-Mobile.

This past Wednesday, her phone stopped making calls/receiving them. Whenever she tries making a call, it immediately says “call failure”. When someone calls her phone, she answers it and again it immediately says call failure. We have been trying to troubleshoot this since Wednesday.

1. We reset network settings. It didn’t do anything.
2. We erased all settings on phone- the phone was able to make and receive calls for about an hour. Then it stopping making/taking calls again.
3. We restored the phone on Thursday and it worked great until Saturday morning when she updated the phone to 12.3.1.

All day yesterday (Saturday) her phone has been unable to make or receive calls at all. We tried restarting it several times, toggled airplane mode on and off, changed the date and time to manual and back to auto, turned bluetooth off, changed the cellular settings from LTE Voice and Data to off, and back on. We called our carrier, T-Mobile, and were on the phone with them for two and a half hours last night. We tried a new SIM, they changed the frequency of the signal coming to her phone, switched it to a different tower, messed with network settings on their end, and we still couldn’t get it to work. The phone will just not make or receive any phone calls.

Some things to note:

1. It plays music/sound just fine from both speakers.
2. Voice memos do not work. The screen turns gray when trying to record.
3. Siri works. We can speak commands to Siri and it will respond.
4. Taking a video captures sound as well.
5. We cannot make calls from the Apple Watch she has paired with her phone either (it will say call failed).
6. Facetime Audio does not work, nor does Facetime Video (it will say call failed).
7. Surprisingly, we were able to make one Snapchat call from her phone last night and it went through. It didn’t work a second time.

Does anybody have any idea what is going on here? The issue is only when trying to make any sort of voice communication to another device. I’m afraid the 12.3.1 update screwed up the phone again. We have tried everything with both WiFi and LTE. Nothing works.