Assuming you are doing release to switch here but in either case whenever a caller hears a ringback they are getting it from a device other then Unity or UC.
The transfer out call will be a regular transfer like a phone from CM's perspective.
You call Unity, Unity hits transfer, dials the number and then hits transfer again. At that point Unity is out of the loop until someone routes the call back into Unity.
If you did a supervised transfer you'd just hear the pretty music before the call got pulled back into Unity. That might be a more stable solution if you don't need to hear the ringing.
Sorry not much I can offer from the Unity side here, the order to play ringing would come from whatever PBX (CM in this case) you were using in this situation not Unity.