We propose a protocol to interconnect the GSM mobile communication networks and PSTN through an IP based backbone. The basic functions of mobile communication, such as call setup, call recovery, and handover, together with further applications such as conference calls, are taken into account. Not only the establishment and closing of a conference call are discussed, but also the recovery procedures due to the failure of handover are also considered. Different feasible interconnection types are also studied. There are several possible connecting topologies, and they can be divided into two categories: (1) MSCs (mobile switching centres) of the same operator are connected in traditional architecture-only the GMSCs (gateway MSCs) of different operators are connected via the IP network; (2) all MSCs are linked through the IP network. We have studied the mechanisms suitable for the above connections.