I have had the same issue since updating to IOS14, constant drop outs - no service. Carrier sent a new SIM which didn't solve the issue. Spoke with an Apple engineer over the phone who ran diagnostics over wifi, concluded it was a hardware/baseline error, took it to the Apple store they ran the same test in-store but cellular network was up and the error was not reproduced there. They kindly replaced my iphone XS max considering the remote diagnostic test failure. Took the new phone home and the same problem is still happening. I am now using the temporary solutions found in this thread to keep a stable cellular connection: Turn off 4G & VoLTE, use 3G only. Turn Wi-Fi calling off. This works for me although it's a bit slower downloading but at least I'm not missing anymore calls. Hopefully apple are working on a fix for this even if they aren't acknowledging the issue. Surely they are aware of it by now?