No ring back tone, or double ring back tone heard when calls are made.
There is a setting in the phone to make a decision on whether to deal with the 180 SIP message received after the 183 SIP message.
The setting is “180 Ring Workaround”.
You can configure this via web or provisioning.
Via web GUI: Features->General Information->180 Ring Workaround
Verify the setting is set to "Enable"