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