I am re-posting this thread as another had to be removed and the answer was later found...
We have home-based reps that do not have a physical phone in an office but use Mobility. They are set up with a 7941 device with a dummy MAC like SEP003000000085 (sequentially numbered) and an RDP. We also tried setting them up with a UDP. We were not getting ring-back only in Scenario 4.
Scenario 1: Internal calls to their 4-digit extension = OK
Scenario 2: Outbound call from office phone to their full 10-digit DID number = OK
Scenario 3: Inbound calls to a toll free number and Exchange UM auto attendant, selecting their 4-digit ext = OK
Scenario 4: Inbound calls to their full DID number = NOT OK. Dead air to caller, no ringing, but audio is fine once answered.
We found out from Cisco that it is bug CSCtt15145 with same scenario reported in CUCM version 8.5(1.12900.7). Setting the delay before ringing on the RDP to 3 solved the issue.
https://tools.cisco.com/bugsearch/bug/CSCtt15145
Symptom:
Unable to blind transfer a call to a remote destination because CUCM does not forward the 180 ringing or 183 Session Progress back to the original calling phone attempting to blind transfer the call.
Or
No ringback on calls to a remote destination when either the internal IP phone that is associated with the remote destination is unregistered or not configured.
Conditions:
The delay before ringing timer on the remote destination that rings is set to "0" and the associated internal phone is unregistered.
Workaround:
Set the delay before ringing timer on the remote destination to a value greater than "0".
