We get this question all the time.
Hopefully Mitel fix this in a future version.
My impression so far is the only way to prevent this undesirable behaviour is to forward the call to a Hunt Group of type NameTag, but it's a bit of a rigmarole to setup, and not very practical to setup for hundreds of users.
I try to convince the users to fix it by creating a rule in their email inbox to forward the voicemail messages when they arrive. The users are rarely impressed by this.