Three-network integration transitional voice communication method and three-network integration transitional signalling system

The invention discloses a three-network integration transitional voice communication method and a three-network integration transitional signalling system. The system comprises a number integration signalling subsystem for generating a pseudo-public switched telephone network (PSTN) number at the internet protocol (IP) end by taking a PSTN number at a PSTN end of an international standard serial number (ISSN) user terminal, so that the pseudo-PSTN number is used as an identification number for establishing interconnection and intercommunication call between the ISSN user terminal and a PSTN telephone network user terminal, wherein the ISSN user terminal comprises an ISSN terminal signalling platform, and also comprises a sharing user gateway signalling subsystem, a crossing network strategy signalling subsystem and a user gateway signalling subsystem. By the three-network integration transitional voice communication method and the three-network integration transitional signalling system, the ISSN user terminal and the conventional PSTN telephone network user terminal perform interconnection and intercommunication of two-way call on the voice level in a three-network integration communication system.