If you are using an auto attendant good luck. It will always ring a single ring as it has been picked up already by the system unless using a console.
System options - Ringing Cadence should be set to external. Although set to internal, I had the same result, don't know if you need to reboot.
"Ringing - Discriminating
Description
Discriminating Ringing lets you distinguish between incoming internal calls, incoming trunk calls, tie line calls, and Callbacks by means of different ringing patterns (cadences).
The ringing patterns for Ringing - Discriminating are:
Incoming internal: 1.0 s on and 3 s off (internal ringing)
Incoming trunk: .5 s on, .5 s off, .5 s on, 2.5 s off (external ringing)
Callback: .5 s on, .5 s off, .5 s on, .5 s off, .5 s on, 2.5 s off.
Conditions and Feature Interactions
None.
Programming
For tie line calls, enable Ringing Cadence for Tie Line Calls in the System Options form.
Operation
None.