Checking Missing Assignments 
You have discovered that no user could be determined for an inbound call. You can check why the call was not assigned by using this procedure.
Note
You can only carry out this procedure for the last call in the system.
Call SAPphone administration (SPHA).
If other calls have entered your system in the meantime, repeat the call or simulate it by choosing Simulate inbound call.
Select
Check the data and information displayed. Also read the associated long text.
Choose Call/Log.
The inbound call is simulated again. However, each step in the search for the caller is logged this time and, as a result, the step in which the error occurs can be easily identified from the log.