A single end user reports that the message waiting indicator lamp on his 7945 IP phone does not light when messages arrive or upon incoming calls. Which fault domain should be investigated first?

Voice gateways handle PSTN call routing and would not selectively affect the MWI lamp of a single IP phone.

CUCM Media Resources such as MOH servers or transcoding DSPs manage audio streams, not voicemail notification signaling to a specific phone's lamp.

Since the issue affects only a single end user's phone, the most likely source of the fault is the IP phone hardware or its local configuration. The MWI lamp on a Cisco 7945 is controlled by the phone firmware responding to SUBSCRIBE/NOTIFY signaling from CUCM; a lamp that never lights suggests the phone itself may be faulty, misconfigured, or running buggy firmware. Investigating the IP phone first follows standard fault isolation - start at the device closest to the reported symptom.

Network routers and switches are infrastructure shared by many devices; a problem there would typically affect multiple users, not a single phone's indicator lamp.