I tried to sort exactly the same issue out with my m2. Sony support suggested reloading firmware so I did. No difference. They then felt that Vodaphone was the problem. Vodaphone refused to accept responsibility and told me to return to Sony.
So the problem went around in circles. Let me say, Sony support were fine and the contacted me but the problem wasn't ever fixed.
Audio on telephone calls through the phone earpiece is very low. It both sends and receives at low volume. The problem is compounded by the fact that my wife also has an M2 so it's difficult to work out what we are saying to each other.
I have not been happy with this aspect of the phone at all for some time and have been waiting for some kind of update. It does everything really well except make phone calls.
So, my only answer sadly, was to buy another's phone.
What concerns me about issues like this is that as a customer, you are sent back and forth between the manufacturer and the mobile phone company. Ultimately, the problem was never fixed.
I was very disappointed with the M2 to say the least so it's no longer used.
