Patent ID: 8374079

Claim:
A proxy server comprising: a processor; and a memory; wherein said processor executes, at least in part by using the memory, an SIP proxy server function of intervening an SIP message transmitted and received between a user terminal, and a working SIP (Session Initiation Protocol) server and a spare SIP server; a message kind determination function of determining a kind of received message; a transfer destination failure detection function of detecting and notifying a failure occurring on said working SIP server; a call failure encounter determination function of determining, based on a notification from said transfer destination failure detection function, whether a message received by said proxy server function belongs to a call flow encountering a failure of said working SIP server or not; and a destination setting function of setting, based on a notification from said transfer destination failure detection function and according to a failure occurrence situation of said working SIP server and a kind of message received by said proxy server function, a transfer destination of the message, wherein said call failure encounter determination function comprises a function of using a call identifier list which holds a call identifier stored in said SIP message externally received and is once erased all at the detection of failure occurrence on said working SIP server to determine that said message holding a call identifier not recited in the call identifier list encounters a failure of said working SIP server.