Watch out with the solution (or trick, or kluge) above.
Firstly let me explain why it appears to solve this issue, it's pretty simple.
In later software, if a User gets a voicemail message, it will light the lamp on ALL of that Users DEE associated phones. For example if the mailbox linked to their Main Desk Phone gets a message, it will also light the lamp on their Home IP phone, and their other desk phone etc.
In the case above, the User is a phantom and its associated phones were actually not other phones that belonged to the phantom, but instead they belonged to other people in the support team.
However....
When an OAI application downloads data from the 5000, it can get confused as to who owns which phones. This becomes apparent when laid out like this:
Support User 1 - Owns ext 1001
Support User 2 - Owns ext 1002
Support User 3 - Owns ext 1003
Phantom User - Owns ext 1001, 1002 and 1003
If the OAI application tries to use obtained User and DEE data from from the 5000 to determine who owns which phone, it can cause problems.
I've found that Vmail to Email and the Xarios App Server solution to be good.
As suggested earlier, Mitel ideally need to add a proper way to light lamps on more than one phone for a single mailbox.