@d1986: Is the issue persistent for all contacts, and in all locations? Have you tried pressing the Volume Up button on the side of the device while on a call to raise the call volume? If you have a case or cover on the phone, try removing it temporarily, to check if it is obstructing the speaker.
To test the hardware, open the Phone app and dial *#0283# into the Keypad. Now, select “Start: RCV_1stMic” and speak into your phone like you’re on a call. Can you hear yourself clearly during the playback?
