I found this on a forum XDA-Developers it made my HTC HD2 phone work (not fully tested yet )
Apparently it's a common fault on Jags and Volvos too that the phone pairs but doesn't work.
Took me a while to realise you need to buy a registry editing programme for just over a quid via the marketplace or app store to do the job.

Please note I did just the last bit as per these instructions.

1. Modify the HandsFree plug-in:
HKLM\Software\WIDCOMM\Plugin\HandsFree
i. Change SupportedFeatures to 45 (default is 1135)
ii. Add value HFBDA binary, set to same value as LastConnectedBda
iii. Add value HFDeviceType decimal 2

2. Modify the WIDCOMM bluetooth general settings:
HKLM\Software\WIDCOMM\BtConfig\General
i. Change bdaddr to (no value)
ii. Change DisableReopenAudio to 0 (default is 1)

3. Reboot device

What i did was a hard reset of the phone which reset the registry to default.

Then, I just changed the SUPPORTEDFEATURES to 1085 (which changes to 1081 after a reboot, but I think you have to put 1085 before rebooting. Dont put 1081 directly in). I did NO other registry changes as per those instructions.

Rebooted the phone, then realised it wasnt constantly disconnecting/connecting. Checked the car display and ALL contacts had been transferred.. and the connection is stable.

Hours wasted testing changes, but finally got there.

I posted this fix on the XDA website back in 2009 and have been living with that 'fix' since - as most of my out going calls are to my wife, I merely programmed the cars voicemail shortcut to the wife's number!

BUT now thanks to yourself, changing the SupportedFeatures to 45 has resolved the problem at long last!

I wonder how much longer I will keep that damned phone! Microsoft phones are notorious with their bluetooth connections - I had the same problem with my Nissan Pathfinder!