Bah another problem has reared it's ugly head...
In our office we have been successfully been using Dual-Mode mobile phones on our CallManager system. Basically this is either a Nokia E60 or E65 phone running SCCP software provided by Nokia, which uses Wifi to connect to CallManager. When calls come in to the DDI the Nokia has a shared line that also rings out at the same time so you can answer the call on either device - great so far!
The problem is that when the DDI gets called, it calls both the GSM line and the SCCP line of the dual mode phone at the same time - which is rubbish.
Documentation on Mobility at the moment appears to be incredibly poor. :(