NAT

To correctly identify and translate voice traffic flows, a NAT device must inspect Layer 3 and Layer 4 protocol and port information, and sometimes information embedded in the packet and protocol message payloads.

Cisco IOS NAT ALGs can support a number of voice signaling protocols, including H.323v2, H.225, H.245, RAS, SIP, and SCCP. If you modify the port used by SCCP in your network, however, you must explicitly configure the new port using the ip nat service skinny tcp port port-number command on the Cisco ALG.

Finally, note that NAT does not support voice signaling encryption.