Aastra 9000i Series Administrator's Manual page 224

RPORT
The Session Initiation Protocol (SIP) operates over UDP and TCP. When used with UDP,
responses to requests are returned to the source address from which the request came, and returned
to the port written into the topmost "Via" header of the request. However, this behavior is not
desirable when the client is behind a Network Address Translation (NAT) or firewall.
A parameter created for the "Via" header called "Rport" in RFC 3581, allows a client to request
that the server send the response back to the source IP address and the port from which the request
came.
When you enable "Rport, the phone always uses symmetric signaling (listens on the port used for
sending requests.)
An Administrator can configure "Rport" using the configuration files or the Aastra Web UI.
Configuring Rport Using the Configuration Files
Use the following procedures to configure Rport on your phone.
Configuration Files
For specific parameters you can set in the configuration files, see Appendix A, the section,
page A-32.
41-001343-01 Rev 02, Release 3.2.2
Note: Configuring the Rport parameter is recommened for clients behind
a Network Address Translation (NAT) or firewall since this parameter
allows a client to request that the server send the response back to the
source IP address and the port from which thre request came.
"Rport Setting" on
4-61