Abstract: According to one embodiment, there is provided a telephone system comprises an exchange apparatus to be connected to a best-effort type network and a telephone terminal to be connected to the exchange apparatus. The exchange apparatus includes a first interface unit which is connected to the best-effort type network, a second interface unit which forms a communication route differing from the first interface unit, a detection unit which detects deterioration in a quality of service of a call during establishment between the telephone terminal and its partner via the first interface unit and the best-effort type network, and a call moving unit which moves the call to a communication route passing through the second interface unit when the deterioration is detected.

Abstract: An SIP server continuously monitors the registration information of each of SIP terminals in a terminal registration state table. When the registration state of the SIP terminal is changed to “unregistered” as the Expires date is exceeded or a failure occurs in the SIP server, the SIP server transmits a REGISTER request message to the SIP terminal. When REGISTER comes from the SIP terminal in response to the REGISTER request message, the SIP server again sets the registration state of the SIP terminal in the terminal registration state table to “registered.