We've seen this same issue recently with a iPhone 6. The microphones would work fine in voice memos and other apps, however on a phone call the microphones ceased to work.
Our solution was as follows
- Remove board
- Remove the shield on the back of the board at the top, the one that just peels off
- Connect a temporary charge port, battery and screen and turn on the phone (plug in charge cable to turn it on)
- Make a call to another phone that you have, put on speaker phone.
- Whilst in call lightly apply pressure to the U1202 Power Management IC, the shiny one with a Apple logo.
- Talk into mic whilst applying the pressure, the mic may start working or you may hear crackling noise. If it does you know that this chip is probably the culprit
- Reflow the U1202 chip and test again, it may just work.
We tried the above and it fixed this exact issue.