Secure Computing SG300 User Manual page 140

If you use an external SIP service such as the Gizmo Project or Skype, you typically do
not need to use the SIP proxy. These services use STUN (Simple Traversal of UDP
through NATs) to facilitate communication from behind a masquerading firewall.
The SIP proxy listens on UDP port 5060 for SIP requests (this is the standard SIP port
and should not generally need to be changed), and UDP ports 7070-7079 for RTP traffic
(the actual voice data).
Configuring the SIP proxy
Check Enabled.
The Outbound Interface is typically the LAN interface. This is the interface connected
to your private network on which SIP clients are located.
The Inbound Interface is typically the Internet interface. This is the interface through
which remote SIP clients can be reached.
Click Submit.
Details of incoming and outgoing calls are logged under Diagnostics > System Log.
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Network Setup