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If you have the following configuration:
IP Phones<-LAN->Asterisk<-LAN->PSTN Gateway<-->PSTN
then you can direct all outbound calls to the PSTN through the PSTN gateway. The IP phones can use the private IP address of the Asterisk to perform SIP registration. You can then set up the dial plan on the Asterisk to route all outbound calls to the PSTN gateway to access the PSTN.
If everything is IP as follows:
IP Phones<-LAN->Asterisk<-LAN->Firewall + NAT<-Internet->ITSP
Then all outgoing calls can be routed to the ITSP over SIP. The IP phones can use the private IP address of the Asterisk to perform SIP registration. You can then set up the dial plan on the Asterisk to route all outbound calls to the ITSP account. Note that if you¡¦re using a NAT that you¡¦ll likely have to configure the Asterisk and Firewall + NAT device to allow SIP and RTP traffic to get through. You may also need an external application layer gateway to convert the SIP signaling depending on what Firewall + NAT you are using.
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