If it says unauthorized I would have to assume the password you used for authenticating is not the same password set in Axon for that extension.
What does Axon report in the logging window? If the username or password was wrong, it should make some sort of indication that an unsuccessful attempt was made to register extension X from IP Y.
Also if by 'IMS' you are referring to your P-CSCF app, I suggest you distinguish this in the future because NCH has it's own software called IMS built to work with Axon that has an entirely differently purpose (on-hold music), so it is quite confusing when not distinguished.