On Pre-Paid plan, can't call.

On what phone? 

Run the imei

https://www.att.com/buy/byod/identify?devicetype=phone

On Xiaomi POCO X4 Pro 5G

IMEI 867035053221386

i dont beleive the Xiaomi POCO X4 Pro 5G is on the at&t supported list, @formerlyknownas can confirm 

Dude, run the imei on the byod link, but I can tell you, it says that phone can't work on at&t. 

No phone from Xiaomi is AT&T capable. 

I understand that.

But it was working less than a month ago, that's why I'm asking if there's something else I can try.

I just have been told that my incoming calls are sent directly to voice mail.

Hello, @mchavarin! We understand how frustrating it can be to not be able to make any phone calls!

Based on the information you have provided, let's meet in a direct message to further discuss some troubleshooting techniques. Please check your Direct Message Inbox (it's the chat icon next to the bell icon in the upper right corner of the Forums).

In the meantime, please make sure you have a compatible device that is supported on the AT&T Network since we have shutdown the 3G network.

We look forward to chatting with you soon.

Thank you for reaching out to the AT&T Community Forums!

Rachel, AT&T Community Specialist

Btw, I do seem to be getting SMS still.

@mchavarin this is typical with a non supported phone. Data and text works but calling doesnt, as at&t systematically shuts down the 3g networks phones stop working they have announced this for over 2 years to get a compatible phone or this would happen.

The issue is your phone is not capable of voice over lte. AT&T has been shutting down its 3G network since February 22nd. It is possible the 3G was not shut off in your area until just a few weeks ago. But since it's shut off now, and your phone is not capable of voice calls over AT&T's LTE network your options are to switch phones or switch service providers. That phone won't work on Verizon but it may work on T-Mobile.