A company uses Cisco Unity Connection integrated with Cisco UCM. Users report that when they leave a voicemail for a colleague, the message is deposited correctly, but the recipient's phone does not light the Message Waiting Indicator (MWI). Callers can dial into voicemail and retrieve messages normally, and outbound calls from the phones work. You verify the SIP integration between UCM and Unity Connection is up and message deposit works. What is the MOST likely cause of the MWI failure?