Unlocked Samsung S21 quit working...can only text now

I switched to US Mobile, taking my S10 5G (that didn'r work with AT&T anymore) with me. Easy transfer, everything works now.

Life is too short. You could spend a ton of time trying to debug why your phone doesn'twork anymore, or just switch. Much faster and painless.  AT&T doesn't really deserve your money after this network switch disaster.

We understand the importance of having a properly working device @Opodude.

Since the shutdown of our 3G network, some non-AT&T devices that were ported in are experiencing service concerns, even though they may have been on our list of compatible devices. While we work to get this resolved, here are some tips that have helped some customers in your situation:

• Try turning Airplane Mode on for a few seconds then off again. This will force a connection to the nearest tower.
• Try resetting your network settings. To do this, go to Settings > System > Advanced > Reset options > Reset network settings.
• Make sure your device software is up to date.
• Visit an AT&T store to get a new SIM card.
• Program your device's APN settings:
  • Tap your Settings menu.
  • Choose either Wireless & Networks, More Networks, or Connections.
  • Tap Mobile Networks, then Access Point Names.
  • Find and select the option to add a new APN. You may find a plus (+) symbol on the screen, or tap the menu button.
  • Program each field using the settings found in the table.
  • Save your changes to return to the APN's menu and choose the new APN.

Please let us know if any of these solutions worked for you. Thank you for reaching out to our AT&T Community.

Darais, AT&T Community Specialist

If I add a new APN, I have to fill out blank tables. I have no idea what information goes into these tables. I have tried everything else, but I have no idea how to fill out these tables correctly.

Same problem here. AT&T service says that my phone is no longer compatible!

@Dumbfoundeder , you are forcefully using a 3G network. I was warned that on April 1st, my service will be shutdown eventually.

The problem is with UI 4.1 on unlocked phones.  Samsung borked the CSC  The only current solution to restore making and receiving calls is to flash your phone back to UI 4.0 . Please refer to this thread for the remedy:
https://forums.att.com/conversations/more-att-prepaid-discussions/cannot-make-or-receive-calls-on-non-att-4g-phone-3g-sunset-is-to-blame/6251b822a7e2d409585d269a?commentId=62563942453e2f45c8bcc9c3&page=4