In -call volume suddenly getting too low, that can't be hearable as check the same with speakerphone is okay.
Why the Nokia HMD didn't provide the solution for this issue in Nokia 7.1??
Further, carrier is VoLTE enabled. Definately the baseband or hardware having bug. As the carrier is working without such issues in Nokia 3.1.
Nokia HMD accept the issue with Nokia 7.1 and hope this would be resolved soon.