Chosen Solution
When I call, I hear what a person says, but he does not hear me, so where the problem is? Sorry for bad english.
The problem is likely with the microphone (s), but it could also be a software or cellular issue. I suggest running a troubleshooter, updating the firmware, and contacting your telco. If none of that works, I’d suggest a reset of the device. If that doesn’t work, your only option is to make claim under warranty or consumer protection laws, unless you want to fix it yourself.