I have been using my HTC Velocity the same way for a considerable time, and just today I noticed that it has started to malfunction.
Traditionally when I have been on a call, and the call has gone on for some time, the screen 'turns off' - this is normal! Normally I would then press the power button (at the top of the phone), and the screen would 'turn on'. I would remain connected to the current call, and I could then use the phone to check messages, email, etc.
However now when I press on the power button the phone hangs up my current call.
I am wondering if there is anyway of fixing this?