Same issue here, low volume while in calls. But that is the least of my problems. Lately I've been getting pure white noise while trying to make calls. It's so loud I can't hear the person on the over end of the phone, and they can't hear me. It's random right now but happening more frequently so I can't make calls from handset now. When I press the mute button, the noise goes away and I can hear the caller crystal clear, but they can't hear me obviously. Sometimes when I unmute the phone or put it on speaker, the white noise disappears but the caller still can't hear me. It works fine over bluetooth.
For those having the static issue over bluetooth in the car, I had the same issue with my Z10, just move your handset closer to the stereo deck and it should solve the issue.
Bought my phone online so I do t have a warranty/receipt

I hope they fix this with a software update. I have a phone I can't make calls with....
Posted via CB10