iPhone 7 suddenly started giving call failed error

My daughter has an iPhone 7. About a month ago, she started getting errors when trying to make/receive calls and/or make/receive FaceTime calls. Her phone just hangs up and gives the error "Call Failed." I tried everything from resetting the phone, restoring the phone to factory, resetting network settings, and contacting apple support through the chat just to be told there is a physical problem with the phone and it needs to be "repaired/replaced" for $340. I got on YouTube and saw other people were having the same problem that appeared to be around/after IOS 14.5-6 updates. I tried the dial *#31# and miraculously the phone started operating normally again. But, it appeared to be a temporary fix, as the problem returned. I tried other things such as the Cellular data options, adjusting time/date, and other tricks some people suggest along with the dial code. I can do the dial code after several hours between or day(s) between the code and the phone will operate normally just to have it revert back to the call failed error code. I feel this is more a software issue than a hardware issue. I could really use some input to resolve this issue. She is running the latest software, 14.8, so I don't know what else to try to get the fix to stick.