PBX (Private branch exchange) 4 normally routes calls through IP (Internet Protocol) network using VoIP (Voice over IP) with SIP (Session Initiation Protocol) for call setup. The SIP trunk module 45 periodically transmits OPTIONS messages to SIP Proxy Servers 61/62 and monitors 45b the responses. In the event that no responses are received the SIP call control module 45c declares the SIP trunk unavailable and the call control module 41a routes calls via a PSTN (Public Switched Telephone Network) backup trunk 44.
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