I made some test today... Next version will attempt a local MessageSend command if there is no external number configured for a particular extension. You will have to enable at least a phony web based sms service in fop2.cfg to enable the SMS menu.
Not sure if this is the right approach... as you won't be able to send sms to external numbers and internal phones from FOP2, you will have one or the other. And transfer to external will be hindered too.. perhpas I have to add some fop2 config option to enable internal messaging, or perhaps use/integrate FOP2 chat with MessageSend option (so in theory you could chat between a phone and fop2, although this option will require a special messaging context to make it 'compatible' with FOP2 chat, so it will be harder to implement).
So, stay tuned... if you want to test it out (beta test), contact me via the live help and I can send you the proper files to do a field test.