No sound during call

Hello all !

Someone gave me an iPhone 6 on iOS 11.3.1.

On this iPhone 6 everything works just perfect except the sound during calls.

When i call someone, he can't hear me and i can't hear him (even in speaker mode).

Also the head-speaker and the micro are good. I have tested them with the voice memos.

I have also noticed that when someone call me even after i hangout the phone keep vibrating .

I have no problems with jack plug (i have tested it with an app).

I have downloaded a new firmware on Internet and flashed it with iTunes. It still don't work.

Can you help me ?