This document specifies interworking between the Session Initiation Protocol (SIP) and QSIG within corporate telecommunication networks (also known as enterprise networks). SIP is an Internet application-layer control (signalling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include, in particular, telephone calls. QSIG is a signalling protocol for creating, modifying, and terminating circuit-switched calls (in particular, telephone calls) within Private Integrated Services Networks (PISNs). QSIG is specified in a number of Ecma Standards and published also as ISO/IEC standards. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.