I have the same problem; my voice can be heard clearly on the other phone only if I use the speaker or headphone mic.
Genius at Apple Store made a Diagnostic and revealed a broken bottom microphone, even if it is clearly functioning during voice memos, FaceTime audios, and WhatsApp or Telegram calls. If I had not been prepared in iPhone hardware assembly, Genius would have convinced me that the microphone is really broken since he claimed that during voice memos the device uses the front microphone, while this is not true (can be demonstrated starting a voice memo and blowing in front and bottom microphone).
However, he told me that the broken microphone is soldered on the logic board of the phone and the only solution is phone replacement for 339€.
Actually the microphone isn't soldered on the logic board, but on the Lightning Connector module which can be replaced easily; however Apple seems to prefer to ask for 339€ to get a 30€ problem solved; Genius claimed that Apple doesn't offer the module replacement solution to preserve security during battery charge, to avoid the risk of fire and explosions caused by possible errors during the replacement process.
The verdict of a Genius Bar appointment is in any case the following:
Bottom microphone is broken, the solution is the replacement of your iPhone 6 with a new one for 339€.
I am extremely doubtful in a microphone problem here; I found that somebody tried to replace the Lightning Connector module without success and this can demonstrate that the microphone is not the faulty component. I can just suppose there is a signal processing bug in the software or a faulty integrated circuit somewhere in the iPhone hardware that processes the audio signal before it is used in the RF section of the phone.
If you search in this forum, you can find that this problem also affects older iPhones (iPhone 4) as well as the newest iPhone 8.
I am an Apple customer from many years now, but I feel really disappointed about this assistance experience which demonstrates poor transparency and solution choices for the problem.