| 摘要 |
During normal operation, a user terminal supported by an IP-PBX server establishes a call by resolving the name of the IP-PBX server using a DNS server. When the DNS server provides the address of the IP-PBX server, the IP-PBX server can be contacted. However, if the SIP controller learns that the IP-PBX is no longer operating, the information stored by the service provider environment may be used to build or configure an IP-Centrex server at a remote site which takes over for the failing IP-PBX server. The newly configured IP-Centrex server then registers itself with the SIP controller and either the SIP controller or the IP-Centrex server then informs the DNS server of the IP address of the IP-Centrex and tells the DNS server that the IP-Centrex server will respond to messages for the failed IP-PBX server. The newly configured IP-Centrex server then handles the messages and control flow previously handled by the IP-PBX server. When the IP-PBX server is repaired or replaced, the IP-PBX server informs the SIP controller that it is back online and either the SIP controller or the IP-PBX server then informs the DNS server that the IP-PBX server will again be handling the call requests for the IP-PBX server.
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