I had an issue with my screen, so my phone was sent to the service center and I got my screen replaced with a new one. Since then everyone who's calling me is complaining of a very annoying Echo of their voices. Even I tried it myself calling my number from my wife's phone and it was a really bad echo.
I noticed that the problem is less if I'm using the speaker or attaching the AKG headset.
I tried all the solutions that I found on my search i.e. updated the phone, removed the back case, tried it in safe mode, and even did a factory reset but it didn't all work.
They might have not replaced the microphones correctly. The AKG headset has a microphone built-in (IIRC), which would explain why the issue goes away if you plug it.
I suggest you contact the service center to send your phone again.
