Phone keeps vibratig after answer calls.

Hello. I have an iPhone 5S and suddently this thing started to give me this trouble.

I can answer a call as usual, but than the phone keeps vibrating for 10 seconds (time on call already counting) and there is NO SOUND. Not in the earpiece speaker, nor other person can hear me.

This is driving me crazy. I have tried another battery, another charging dock, answer without the earpiece ribbon connected....

I have also restore the phone connecting it in dfu mode to itunes. Still nothing.

Can someone please help me?

Thanks.