> How about just giving me a SIP endpoint that corresponds to my mobile number, and, when there's an incoming call, query my SIP presence, if I'm present, ring

> How about just giving me a SIP endpoint that corresponds to my mobile number, and, when there's an incoming call, query my SIP presence, if I'm present, ring the SIP, if not, ring mobile. SIP also supports SMS if I'm not mistaken. Andrews and Arnold lets you do this today (in the UK): http://aaisp.net.uk/telecoms-mobile.html http://aaisp.net.uk/telecoms-mobile.html