You may have seen this in documentation, but apparently this is not configurable:
Unity Connection Messaging System
This account acts as a surrogate sender for messages from unidentified callers. Thus, user messages from unidentified callers are identified as coming from the Unity Connection Messaging System mailbox (UnityConnection@
The alias for this account is UnityConnection. This account can be viewed in Cisco Unity Connection Administration, but cannot be modified or deleted.
While the name is not editable in Unity Connection, you can create an Exchange mailbox called unityconnection@