The present document specifies the stage three protocol description of the signalling interworking between ISDN DSS1 protocol and SIP based on the concatenation of ETSI ES 283 027 [1]@ ETSI ES 283 003 [5] with ETSI EN 300 899-1 [2]. The concatenation method describes only the SIP/ISDN parameter mapping without ISUP procedures. In addition direct inter-working not supported by this concatenation of these existing inter-working documents will be described.