My environment: UC Manager 126.96.36.19900-3, Unity 7.0.2
SIP integration doesn’t use MWI DN numbers to turn on and off MWI. Unity will inform UCM to turn on and off MWI by sending a SIP NOTIFY to UCM.
To make sure UCM accepts the SIP NOTIFY and update the phone MWI status, check your SIP trunk configuration. Make sure the “Incoming Call CSS” under your SIP trunk configuration includes the partition of the DN! After setting the “Incoming Call CSS”, go to the Unity admin page, under the subscriber settings, the message configuration, refresh the MWI status. You will the the MWI of the phone lights up!