I upgraded to an S21 Ultra a few months ago. I've been dealing with a problem that seems to be getting worse. Namely, I will suddenly stop being able to receive calls or texts. Usually my wife will tell me she tried calling me but it went straight to voicemail.
If I toggle airplane mode off and on, this fixes the issue and I often times then get a bunch of texts and notifications come through at once. I've tried turning wi-fi off and on, or turning off wi-fi calling but this doesn't seem to help. Only toggling airplane mode off and on has been a sure fix. It is almost like my phone's registration on the cell network gets messed up. Whatever is going on, this is the first phone I've ever owned with such a major problem. Many of us rely on a phone's basic function (making and receiving calls) to work solidly 100% of the time, especially in cases of emergencies!
Any helps or suggestion would be appreciated, thanks! Happy New Year!