iPhone 7+ no audio in phone calls, except through a Bluetooth device

For the last few weeks, when I make or receive a call on my phone, I cannot hear the other person and they cannot hear me, unless I am connected to a bluetooth audio device (Apple Watch, headphones, car bluetooth). It is not a location / reception issue, because if I connect to the call from the same location on a bluetooth device it works.

I have tried all proposed solutions from turning on airplane mode, turning off blue tooth, automatic routing in accessibility, to resetting the phone.

My wife and I bought our phones, also 7+, at the same time, and she is experiencing the same difficulties despite having completely different apps and settings on her phone to those on my phone. We both have Apple Watches (v3).

There are multiple discussions on this issue, one with over 2000 registered users who say they have the same problem, and yet there appears to be no solution offered. I really hate the idea of Apple not looking after it's customers in an honourable way, but I do remember that Apple was caught shortening the battery life of older phones through a software update. Please, please Apple, let me, and the thousands of others experiencing this issue, downgrade to an older iOS, solve the issue with an update, or at least recognise this as an issue and provide clear advice - what to do with a phone that does not make calls.