caller can't hear me unless on speaker

Hi Ece,

There is always the chance that what you're seeing is a software problem, and that should probably be the first thing to check before you get into replacing parts. Ultimately what's called a DFU restore is the most complete update possible and if that doesn't do it, then yes, you almost certainly have a hardware problem.

So yes, I would suggest you back up your phone then do a full update and see if the problem is still there before restoring your backup.

Assuming that doesn't do the job, then yes, your instincts were correct in thinking that there might be two microphones. As a matter of fact, there are three. When you're making a phone call, you use the two microphones at the bottom of the phone, and when you're on speakerphone it uses the microphone at the top of the phone.

Given that, your problem would appear to be with the lower microphones. There are two of them located on the charging port flex cable assembly; I've marked them in red in the picture below.

While you can't replace just the microphones, you can replace the assembly they're mounted on. iFixit even has a guide all ready for you to use to get the job done.

iPhone 8 Lightning Connector Assembly Replacement - iFixit Repair Guide

And as far as getting the part, you can buy that right here on iFixit, and support their repair efforts at the same time.

iPhone 8 Lightning Connector Assembly

Good luck, and let us know how it all turns out!