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Most SIP servers can receive calls and then redirect them to another SIP server. Asterisk also has this ability. If one of the SIP servers has an integrated PSTN gateway, then calls can be directed to the PSTN through that server.
To allow SIP phones to receive calls when they are located behind a NAT device, there are multiple approaches. One of the more secure approaches is to use an application layer gateway or a SIP aware firewall and NAT device. An example of a SIP aware firewall is that from Ingate.
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