Access to microphone denied by Whats App on iPhone6 for video calls

My trusty iPhone 6 battery recently failed, so I have just migrated everything onto another "newer" iPhone 6. Everything is fine except for Whats App - the old phone worked perfectly making Whats App video calls, but on the new iPhone the microphone is permanently disabled - but only in Whats App. Bothe the iOS Privacy and Whats App settings are properly enabled, I have deleted and restored the app, but no joy.

I can see from various forums that this has been a problem with the iPhone 6 in the past, but surely it is no longer an issue? Can anyone please help? Thanks!