http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cuipph/7960g_7940g/5_0/sip/english/administration/guide/5_0/7960setS.html
Some general advice on how to get 7940/7960 going in standalone SIP via NAT.
1. Give your 7940 a static IP.
2. Forward ports 5060-5062 TCP and UDP and 16384 to 16399 UDP to the static IP of the 7940.
3. Make configuration file adjustments as follows:Set voip_control_port: 5061
; NAT/Firewall Traversal- 0=Disabled, 1=Enabled
nat_enable: 1
nat_address: “”
nat_received_processing: 1; Start RTP range for media (default – 16384)
Start_media_port: 16384; End RTP range for media (default – 32766)
end_media_port: 16399; Register or Not
proxy_register: 1