The CommuniGate Pro SIP and XIMSS Modules also provide the "far-end" NAT traversal capabilities by detecting requests coming from clients located behind remote Firewall/NATs.
The Modules add appropriate Record-Route and Path headers to these requests and they build Media Proxies to relay traffic to and from those clients.
Note: modern SIP clients support various NAT traversal methods (STUN, etc.). Many of these implementations are quite buggy, so it is often more reliable to switch the client-side NAT traversal methods off, and rely on the CommuniGate Pro SIP Module far-end NAT traversal capabilities instead.
Note: due to the nature of the TCP protocol and the Firewall concept, it is not possible (in general) to open a TCP connection to a client behind a far-end NAT ("near-end" NAT configurations do not have this problem). This means that clients behind a far-end NAT cannot initiate TCP (T.120) sessions.
To solve this problem, you may want to: