Default In call mic problem...

Has anyone had the above issue, sent it for repair under warranty and been told they can't find the problem ?

Mine has been away for a month now and whenever I ask "what's goin on", I get told it's not a hardware issue and they can't find the problem.

Was running ARHD 71.1 when problem was noticed but my sons phone runs same ROM without issue. NOT blaming Rom. Put mine back to, complete, stock and relocked bootloader which didn't resolve it.

Just wondering if anyone else has had the same fault and what it turned out to be ?

Cheers

Edit, it's a UK phone.

Sent from my XT1021 using XDA Free mobile app