the invention makes use of the short message service or unstructured supplementary services data capabilities present in today &# 39 ; s cellular networks as a means of providing call control and presence signalling and , in particular , for providing the means of effecting hand - off at least from / to the cellular domain to / from the wifi domain . sms and ussd were originally designed as engineering channels , but sms has become highly popular as a service in its own right . sms and ussd are built into all existing cellular network and predate the current gprs overlay . in particular , sms and ussd can be used while a call may be active in the cellular domain , and a handset may send and receive such messages , with an additional facility which may be used by the invention of receiving acknowledgements of successful receipt of an outgoing sms by the messaged party — effectively a confirmation from the network . the present invention proposes to use sms or ussd as a means of an adjunct call control and presence signalling in the cellular domain between a handset and the sip domain .