Default Echo to the other phone

Hi I have Z2 D6503 the European version and recently I realised I have a problem - when I speak with somebody the other person hears his worths like echo. It happens whith every call and various phones. Only the other person hears the echo, I dont hear it.
The phone is with the latest software. Hard reset doesn't fix it.
What could be the problem and how can I fix it?