Hi Ralph, in system options ensure that the option Ringing Cadence for Tie Line Calls is set to external.
Also a note from the docs is:
Select "Internal" to indicate that a single ringing cadence is to simulate an internal call or "External" to indicate that a double ringing cadence is to simulate an external call.
Enabling External Trunk Standard Ringback in the COS overrides this setting.
Hope this helps