System and methods for allowing the extension of the ISDN/GSM Call Completion to Busy Subscriber Supplementary Service across a Network Boundary between a general Switched Circuit Network (SCN) using ISUP as call control protocol, and an IP Packet Network using SIP as session control protocol. The methods define a set of new protocol elements to be included in the Session Initiation Protocol (SIP) in order to make it possible to implement the required service logic in SIP User Agents and Clients. The methods also define the set of sequences, timers and use cases that a Network Gateway must fulfil in order to allow users from both networks to invoke the service for calls going through the Network Gateway in question; the Network Gateway represents an end-point for both SCN and SIP domains.