To work correctly with your voice flows, a NAT device must inspect Layer3 and Layer4 protocol ,port information and sometimes the embedded payload.

Cisco IOS NAT ALG can support the following voip protocols :

  • H323v2
  • H225
  • H245
  • RAS
  • SIP
  • SCCP

So if you change the default port number of these protocols, then you have to change it also in the configuration with the following command ip nat service <<protocol>> tcp port <<port-number>>.

Pay attention also that NAT doesn’t support voice signaling encryption