I have this problem as well with my iPhone 8Plus. Apple Support and the people at their Genius Bar have not been helpful at all and refuse to acknowledge there is a problem. I have already changed phones once as no one could hear me, and now with the 2nd replacement phone the same problem has started happening again after several months. Even if I shout down the phone they say my voice keeps fading in and out, to the extent that a conversation is impossible.
I have had various iPhones from the 5 onwards, and have never had any problems. So it's not me. Frustratingly the microphones work when I do the recommended tests/trouble shooting, but when it comes to making calls this phone is USELESS.
I have another appointment on Sunday 2 December 2018 at the Genius bar and will just hope they do something about it or replace my phone (yet again)