My mind is absolutely boggled.

I'll try to make this short.

I have an iPhone 6 Plus. Both microphones seem to work perfectly fine

(I've tested the bottom microphone with Voice Recorder-- super clear.

I've tested the top microphone with Siri and Voice Dictation, understands me every single time)

HOWEVER

Whenever I make a phone call with this phone, the person on the other end cannot hear me whatsoever. This is not a matter of being muffled, this is literally no sound at all. The only sound that comes through sometimes is a crackling noise, almost like ruffling a bag of chips or something.

How can both of the microphones work perfectly fine but then completely not work only during phone calls? Anybody know what is going on?