When I turn off Auto NAT on the phone my phone would appearregister under my private IP address.

Ip phone registration issues. If you dont have one you can enter a broadcast IP address ex. Verify that the phone has registered. TA100 port SIP Status becomes Unreachable Intermittently.

In each phone there is a setting for local SIP port for each account. Enter the IP address of your Syslog Server. I have a new Cisco 8851 phone that wont register.

If you look at account 1 it will be 5060 account 2 will be 5062 and account 3 will be 5064 and so on. Make sure that you enter the required information in these files followed by a factory reset on the IP phone in order to resolve the issue. The IP phone cannot download its configuration file.

Each account in each phone. These are several of the issues that might cause a Cisco IP Phone not to register. Use to unlock your phone and delete the CTL ITL files.

The Cisco IP Phone is not receiving the DHCP server address. This suggestion can usually help in the case you had a power outage or if your Internet connection stopped working for a while. With old extension it registers fine but when the user is changed it does not register.

One or more IP phones failed to successfully register. To eliminate the issue you need some way to make each phone unique from one another. If it doesnt work and before you move in to the troubleshooting process its always best to RESET the phone configuration settings to factory default.