I use T-Mobile. With the new S21, I am periodically losing the wifi calling ability. The setup is wifi calling preferred -- the cellular signal is pretty weak around my home. Wifi calling works great immediately after a reboot. Within a day or less, the wifi calling ability quietly drops -- no error messages. Wifi itself is still perfectly fine, and internet connectivity is not affected. The only way out of this state is either to stop and restart wifi connectivity, or to reboot the phone. The last 4 years I had a S7 and had no issues with it. I transferred the same sim to the S21.

Unfortunately Samsung help has not been all that helpful. They have had me clear cache from a protected mode restart, cleared the cache for the phone app, reset my network configuration, reset all configurations, ... -- but the problem keeps happening. 

I checked with T-Mobile, and all configurations at their end seem to be fine. The only thing that they suggested to achieve parity with the S7 is to select the network config without 5G, and see if I still have the problem. I switched to that config a couple of hours earlier. Waiting to see how this holds up.

Anybody else facing this problem on the S21?