Mobile networks drops as soon as I make a call, but comes back when call drops?

One line on my phone was working perfectly fine. I then tried to unlock it. It unlocked successfully after I got the code from Xfinity. Then I reinserted my Xfinity sim back into the phone right after. Suddenly I am unable to make calls. The bars are there. The 4g sign is there. Another phone on the same account right next to it works just fine.

Here is the specific issue.

As soon as I make a call, while it's trying to connect, after a few seconds the bars disappear. A little x appears next to the bars. The 4G sign disappears. And the phone says mobile network unavailable.

Sometimes immediately after, the bars are back. At other times there is no network for a while. But when it returns, the issue is the same. As soon as I make a call, the network completely drops.

I am also unable to receive calls while the network is available. Mobile care was not able to resolve it. The only option right now I have left is to get a new sim card. I will try that tomorrow.

I have reset the network. Taken out the sim. Restarted the phone. Nothing has worked.

While the network is available, texts work. Data works. Just no calling.