ScottyB, this has been in the pipeline from BT for a while now, but like everything there's a commercial aspect to the products they offer and the mobile operators have been trying to scupper it.
A useful article that may give you more info :
http://www.theregister.co.uk/2005/06...fusion_launch/ It also highlights that it's early days for the service so may suffer from some teething problems (one of which is the limited number and types of handsets available).
Hope this helps.
Jiminy