Specifies the signalling protocol for the support of the Call Transfer supplementary service (SS-CT) at the Q reference point between Private Integrated Network Services Exchanges (PINXs) connected together within a Private Integrated Services Network (PISN). SS-CT is a supplementary service which enables a User to transform two of that User's calls (at least one of which must be answered) into a call between the two other users in the two calls. Also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between Call Transfer and other supplementary services and ANFs. Is applicable to PINXs which can interconnect to form a PISN.