I've got the exact same thing!
The quality is great, but sometimes the volume is just way too loud, almost up to the point where it hurts my ear.
One additional thing is when I get a notification on the background that a message has arrived during one of those loud calls, volume actually switches back to normal.
In my registry incall2 was set to 5. Resetted it to 2, hope it works
Orange SPV classic > Qtek S100 > Qtek S110 > HTC Tytn > HTC Tytn II > HTC Touch HD > HTC HD2