I've had my s21ultra since launch with no problems, over the past month though I've differently been getting terrible mobile phone signal. This seems to go from full bars down to 1 to a dreaded 'no service' circle icon with a line through it. Infact, the very act of trying to make a phone call or send a text sometimes kills the signal itself. Or even, it sometime tries to send me a text or can kill my signal. I've tried my sim in other phones, and tried other Sims in my phone, everything points to the phone. Tried network reset, tried loads of mobile network seeing changes, someone's switching airplane mode on and off helps for a while. It can go for a few days being really stable, then have a fit of the network going off, on, off, on, off, on etc. All that also burns down my battery pretty quickly.
Dreaded circle
😔
@Majorj0nny: I'm sorry to hear that you're experiencing this issue. Do you have access to another compatible phone that you can try your SIM in, to see if the issue persists? This will help to determine if the issue lies with the phone, or your network/SIM.
@Majorj0nny: If the issue has persisted with multiple SIM cards, and you have tried your SIM in another device, then there may be an issue with the phone's antenna. If you have a case or cover on the phone, try removing it to see if you notice an improvement. If not, please reach out to our Help Desk on 0333 000 0333, or on Facebook or Twitter, where an advisor will be able to arrange an inspection for you.
Get this randomly. Doesn't seem to be location based, was even wondering if it was when I move the phone or shake it it caused it to drop but then it'll be fine again and I can't change it by shaking it again. 
