Microphone audio stopped working only for phone app

[Android Central Question]

I have a Moto x pure 16GB (XT 1575). The microphone is only intermittently working for the phone app. It will also only randomly work with a headset plugged in. It WILL, however, work when recording video, with OK Google commands, etc. Basically, it will work reliably and everywhere BUT with the phone app. Based on this info, there likely isn't physical damage to the microphone. Audio in video recordings is crystal clear. When the mic does intermittently work for calls, its only for a few seconds. This is not the case with any other app, such as camera/video. It will continue to work with any other app the requires audio.

So far I have tried:

Resetting the phone to factory settings with no change
Clearing the system cache
Disabling ALL apps
Purchasing a pair of earbuds with a built in mic
Removed/replaced the SIM card
Powered down the phone multiple times
Restarted in Safe Mode

The phone has not been dropped, exposed to water or dirt, or abused in any way. Always in a case with a screen protector. It literally worked on one call, then 5 minutes later it wouldn't work.

 

[Rukbat]

Try installing a different dialer app. If that works, it's the stock dialer app that's bad.