This is a common issue with the s21 so you are not alone. I've spoken to my carrier, Tmobile, and everything is fine on their end. It's definitely a bug that needs to be ironed out between the two companies (TMo and Samsung) because something isn't working as intended. I have the same issue as you, my wife, and other people on this forum have mentioned the same problem.
The best thing to do is submit an error report through this app so Samsung is constantly made aware of the problem. This will hopefully help us get a fix sooner than later.
I just switch from Samsung Galaxy 7 (old and cracked) to a brand new "shinny" S21. My "old reliable" NEVER dropped a call. The brand new S21 did for EVERY call over ~7 minutes, since I got a week ago. Today, in a business call it dropped 5 TIMES! I have At&t. The cell signal is poor, but I use WiFi calling and NEVER had a problem before. Is the problem with At&t or Samsung S21, or both? Where should I
If you are experiencing network service issues with calling, SMS or MMS messages, network signals, mobile data, or hotspots after resetting the device, you will need to contact your carrier.
If you complete all of the troubleshooting and are not having issues with the network, such as mobile data, texting, etc., then it would be recommended to have the phone evaluated by a certified technician. You can use the following link to find a list of service centers in he U.S. that can do the repair: http://www.samsung.com/us/support/service/ or by contacting customer support at 1-800-726-7864.
