Mic issues while calling

Hey guys,

I’ve came across many iPhones with this issue:

While calling, people cannot hear me (bottom mic), or very quietly.

Using the speaker function (front mic) the problem is not always fixed.

I’ve tried a software reset, replaced the dock connector with bottom mic and front flex cable. I even replaced the receiver / speaker / display (all original Apple parts).

Seen 100+ iPhones with this issue and cant seem to find the solution.