iPhone 6 -random issue- callers can't hear me
This happened to me too on iPhone 6 plus for a while - I don't necessarily have a solution, but I do have a process that's slightly less annoying than a factory reset. 

When you reboot - I assume you are just switching off &amp; back on? Apologies if this is already how you're rebooting, but just in case it's not:

Instead, try quitting all your apps, then press the home &amp; power buttons at the same time &amp; hold down for about 10 seconds - it will reboot, but it's a hard reboot, and I found that this got rid of the problem for longer than a regular off &amp; on reset. 

I've also found that it comes &amp; goes with software updates, and haven't had it for a while. Fingers crossed that today's IOS 10 doesn't have the issue!