Be aware that your phone has two microphones, one in the top module (for videos and when putting a call on the speaker), and one in the bottom module, for regular phone calls.
From your first post I’d say you have issues with the bottom module.
I would try to disassemble the phone to take out the module and clean the contacts. If this doesn’t help, I’d contact support, tell them what you have already tried, and get a replacement bottom module.
You may also have a look at these threads:
It appears some users had similar issues with the microphone.