HTC release Bluetooth fix for TyTN II, Touch and S710
I haven't seen this issue myself but if you are (according to HTC) experiencing dropped calls occurring when Bluetooth is on then you are in luck as HTC have released a fix for the HTC TyTN II, Touch and S710.
Luckily it's a CAB file so no need to reflash your device and your data should be safe.
You will need to login to the HTC Eclub website to get download it