This International Standard specifies signalling interworking between QSIG and the Session Initiation Protocol (SIP) in support of basic services within a corporate telecommunication network (CN) (also known as enterprise network). QSIG is a signalling protocol that operates between Private Integrated services Network eXchanges (PINX) within a Private Integrated Services Network (PISN). A PISN provides circuit-switched basic services and supplementary services to its users. QSIG is specified in other Standards; in particular [2] (call control in support of basic services), [3] (generic functional protocol for the support of supplementary services), and a number of standards specifying individual supplementary services. NOTE The name QSIG was derived from the fact that it is used for signalling at the Q reference point. The Q reference point is a point of demarcation between two PINXs. SIP is an application-layer protocol for establishing, terminating, and modifying multimedia sessions. It is typically carried over IP [15], [16]. Telephone calls are considered a type of multimedia session where just audio is exchanged. SIP is defined in [10]. As the support of telephony within corporate networks evolves from circuit-switched technology to Internet technology, the two technologies will coexist in many networks for a period, perhaps several years. Therefore, there is a need to be able to establish, modify, and terminate sessions involving a participant in the SIP network and a participant in the QSIG network. Such calls are supported by gateways that perform interworking between SIP and QSIG. This International Standard specifies SIP-QSIG signalling interworking for basic services that provide a bi-directional transfer capability for speech, DTMF, facsimile, and modem media between a PISN employing QSIG and a corporate IP network employing SIP. Other aspects of interworking, e.g., the use of RTP and SDP, will differ according to the type of media concerned and are outside the scope of this International Standard. Call-related and call-independent signalling in support of supplementary services is outside the scope of this International Standard, but support for certain supplementary services (e.g., call transfer, call diversion) could be the subject of future work. Interworking between QSIG and SIP permits a call originating at a user of a PISN to terminate at a user of a corporate IP network, or a call originating at a user of a corporate IP network to terminate at a user of a PISN. Interworking between a PISN employing QSIG and a public IP network employing SIP is outside the scope of this International Standard. However, the functionality specified in this International Standard is in principle applicable to such a scenario when deployed in conjunction with other relevant functionality (e.g., number translation, security functions, etc.). This International Standard is applicable to any interworking unit that can act as a gateway between a PISN employing QSIG and a corporate IP network employing SIP.