To establish a Bluetooth data connection via your phone it has to support the Bluetooth DUN profile, I know the HTC device doesn't offer this capability but if its running WM6 or WM6,1 this profile can be added.
I have added the CAB DUN profile installer to this post, all you need to do is unzip it ONCE, this will provide a single file with the extension of .CAB copy this to your HTC phone then use File Explorer on the phone to locate it and tap it once to install the file. Answer OK/ Yes to the stupid warning messages then once its finished press and hold the power off button until the device says you might loose data, press OK and switch on the phone - The re-boot is required to make sure the settings take effect.
Now delete the HTC from the TomTom and delete the TomTom from the BT pairing within the HTC, next establish a new connection and it should all work - NOTE THIS IS ONLY FOR WM6,0 AND WM6,1 - Mike