Hi Liam
Many thanks for your help with this one I've tested it all this morning and can confirm it all works fine. We can receive calls through to the phone and through the desk. The radio guys have tested all their parts so can take the calls through the handset and mic and do all the recording in to a cart etc.
So for anyone else's info that may have the same/similar setup as us you can get telephone calls through to the studio desk without a dedicated BT analogue line. For a bit of info we have an Avaya IP phone system and have managed to do this so it is possible.