Application switch for controlling session initiation protocol communication via at least two subscriber's lines and method of operating the application switch

An application switch controlling an SIP(Session Initiation Protocol) communication by using two and more subscriber lines and an operating method thereof are provided to perform a seamless SIP call connection, more stably and efficiently, by detecting a fault at plural lines connected to an external network including an SIP server and controlling process of the SIP call connection of a communication terminal allocated at a line, where a fault is detected, via other line, where a fault is not detected. An application switch controlling an SIP(Session Initiation Protocol) communication by using two and more subscriber lines comprises the first communication port(210), the second communication port(260), a memory(220), an SIP connection controller(230), a fault detector(240) and an address updating unit(250). The first communication port(210) is connected to an external network including an SIP server via plural lines wherein the first communication port(210) has two and more global IP addresses. The second communication port(260) is connected to an internal network including one and more communication terminals. The memory(220) maintains an NAT(Network Address Translation) table where an SIP address, a private IP address and a global IP address corresponding to one an more communication terminals are recorded. The SIP connection controller(230) converts the private IP address into the global IP address and vice versa by referring to the NAT table, and controls the SIP call connection via the SIP server. The fault detector(240) detects whether a fault occurs at the plural lines. The address updating unit(250) updates the first global IP address given to the first port of the first communication port connected to the first line, where a fault is detected, by the second global IP address given to the second port of the first communication port, where a fault is not detected.