For a phone to receive notification of message events, it first has to send a subscribe message to the sipX server. If a phone sends a SUBSCRIBE for the message-summary event package, it will get notification (see . Enabling MWI Notification).means giving a user VM permission
There are phones that don't send the SUBSCRIBE (i.e. Cisco), but will still present an indication if they are sent an unsolicited NOTIFY (an early version of the internet-draft apparently allowed for this - it is not a part of the RFC, and don't let phone vendors tell you it is - they should SUBSCRIBE). There is some provision for this in the sipXregistrar; it is a hack, but it sometimes works. I don't think there is a GUI for configuring it (config server people?), but you can edit the file:
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