I am using a desire HD which isnt in the compatability list,...
I guess, thatīs most likely the problem.
Usually, supported phones donīt disconnect from the HFT, even if you dial a number unknown to this system and even if you use the phone itself for dialing.
As for the HFTīs 50 numbers limit, did you already know, you can use not only the HFT but the nav system, too, to make any calls?
Itīs not as comfortable as the HFT, but still it works.
Start from the main menu of your nav system.
If you say "Phonebook" (using the steering buttons for the nav system, not the ones for the HFT), you will find an option to transfer the whole phonebook into the nav system, including all contacts beyond the HFT limit.
Keep an eye on both the nav system and your phone, in case there are any messages to be confirmed.
After that, starting from the navīs main menu, say "Phonebook" again and you will find a contact list just like the one in your phone.
Navigate to the wanted contact by saying "Next" (or "Down") and say the according number you see at the beginning of that line.
Then say "Dial" to get connected.
Btw.: Of course, you will have to press the navīs steering button for each of your voice commands.