iPhone X call quality issues

Hi

I seem to be having random issues with call quality. I’ve had several people now tell me that when calling them they can’t hear me or I sound like I’m underwater.

Problem is it seems to be very random.

I have tried 2 different cases (the second seems to be better) but I have also had the problem with no case on the phone.

The other day I was on a call which was fine. I got put on hold so put it on speaker and put the phone down. When they came back I took it off speaker but they couldn’t hear me. I tried tapping the back of the phone hard at the bottom and then suddenly they could hear me.

I’m now worried that I may have a loose microphone connection but I just cannot seem to replicate the issue.

I will book a Genius Bar appointment but I can’t get one for over a week so thought I’d do some back ground Work first.

Thanks in advance for any help.