Chosen Solution
So I brought my iphone to authorized repair center to have my lightning port + headphone jack assembly replaced. after a while I noticed that whenever I receive a call the person on another end can’t hear me so I tried voice memo and it won’t pick up any noise unless I shout closely to the bottom of the phone. front facing and back microphone are working fine when I am making video.
Sounds like the microphone that the “authorized repair center” installed has failed already. They probably used cheap parts and/or did a sloppy job installing the assembly which resulted in damage to the assembly. You can replace the assembly with a more quality part at the link below. I also included a guide for you just in case you wanted to DIY. iPhone 6s Plus Lightning Connector and Headphone Jack iPhone 6s Plus Lightning Connector and Headphone Jack Replacement
