| 摘要 |
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."
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