Enter user programing mode my entering #9876

Press the intercom key.
The 1st option is to change the internal ringing. If you press hold you can modify the ringing for external calls.

Your options are 01 or 02 (typically for internal call cadences) or 11 through 18.

Press the same intercom key to save.

If the phone having line keys that ring for an incoming call, then you would want to change the ringing of the line keys instead of the intercom key.