The only thing that could be a problem relating to any settings is the mute button is selected on the call 😄 but I'm sure that isn't the problem here. Try a network reset. You won't lose any data. Settings >> General Management >> Reset >> Reset Network Settings - follow any on screen steps, after it's done your phone should restart automatically, if it doesn't restart it manually.
If you're in the UK with any S21 series model you should have this update available now. The only reason you wouldn't is if your phone is on a contract, example pay monthly via a UK carrier EE, VOD etc.
I am on a contract with EE. but surprisingly phone is not locked. I think, as it was a preorder and delivered on the of launch, may be was directly from samsung so its not locked to EE. Still waiting on the update though
It's quite rare you would get a carrier locked Dual Sim smartphone. It will have EE's Carrier firmware that's all, EE are usually the faster of the 4 carriers to approve updates and this update is around 10-14 days later than usual after having a couple of problems. If you go to Settings >> About Phone >> Software Information and scroll down a little you'll see the section "service provider software version" and it will end with something like "EVR/EVR,EVR/EUX/EVR. UK "stock" CSC is BTU and occasionally XEU but personally I haven't seen that CSC since S10 series.