This feature allows the IMG to interact with SIP Proxy Servers and Session Border Controllers as intermediate routes between domains. The IMG can route SIP traffic to these SIP entities (SIP proxies) and with the knowledge of their final destination (remote SIP UA).
A SIP Proxy receives SIP requests from a client, even though it may not be the server resolved by the Request-URI. Typically, a UA is manually configured with an outbound proxy, or can learn about one through auto-configuration protocols.
You can configure one SIP Outbound Proxy Server for each external SIP gateway using the SIP Proxy pane under a SIP Profile and then assign that profile to the External Gateway.
RFC 3261 Session Initiation Protocol
RFC 2543 Session Initiation Protocol
Outbound proxy handling (non-redundant)
Outbound registration to outbound proxy
Re-invite and 3xx Redirect to outbound proxy