Patent ID: 8054955

Claim:
A telephone system which realizes voice communication by using a packet network being an Internet Protocol network, comprising: an exchange which accommodates a telephone terminal as its extension; and call processing servers which process calls on the packet network by means of Session Initiation Protocol, wherein the exchange comprises: a first trunk connected to the packet network; a second trunk connected to a public network having a different protocol from that of the packet network; a server status management table configured to be used to register the status of the call processing servers; a trunk status management table configured to be used to manage the status of the first trunk; a monitoring module which monitors the call processing servers, registers the result of a monitoring in the server status management table, and when failures occur in all call processing servers, deactivates the first trunk, the monitoring module periodically transmits inquiry messages to the call processing servers, and if no response to the inquiry message is returned, deactivates the first trunk; and a call control module which acquires the status of the first trunk from the trunk status management table when a transmission request is arrived from the telephone terminal, transfers the transmission request to the second trunk and performs a detour transmission to the public network when the first trunk is inactive.