From the BT FAQs
"If the product is GAP compatible you should be able to register any other GAP compatible handset to the base. However, it is likely that some features will not fully operate as the GAP standard only requires for handsets to make and receive a call, and ring when an internal, external and paging call is received. In the majority of cases, while not mandatory, the handset will also display Caller Display information. There will also be a limit to the number of handsets that can be registered to a base."
I've very recently replaced a Philips system with a BT Freelance. I've managed to register one of the Philips handsets with the new base with no problems or difficulty.