Pondrew, on Feb 11 2006, 13:25, said:
When this used to happen on my original SPV it got progressively worse until the device was fairly unusable (switching to headset mode in the middle of calls). I had to send it to HTC for repair which resolved it permanently.
So although you may be able to temporarily resolve it be aware that the problem may reoccur

Hi,
I had this problem after using a friends Headset.
The switch at the rear of the jack socket had become loose with use.
This meant that the signal to the phone indicating the headset status was not Grounded via the switch.
I worked it a few times with a pointy probe and all was well untill the next time I used a headset, this time
I could not get it going again and upon dismantling the phone THREE of the 5 or 6 contacts from the
surface mounted jack socket had lifted their lands and were not making proper contact with the PCB traces.
I used a temperature controlled iron with a pointy tip, some very thin silver solder and some kynar/mod wire
to bridge each contact to the SMD devices the tracks went to.
You need to be experienced with hand soldering SMD Electronics to do this, so if you wreck your phone don't blame me
I decided I didn't want to risk the switch going again and added a wire link from it to ground, (the centre rear of the socket),
this meant that I could never use a headset but also ensured that the fault would not happen again.
NB: To get to the jack socket you have to dismantle the phone (probably covered elsewhere on this site.
First obtain a jewellers screwdriver set as the screws in this phone are TINY!
Then remove the small screws holding the keyboard PCB in place and unplug it from the main PCB.
This allows access to the special multipin jack socket.
Hope this was of help to someone.