Of course I should have tested it first… problem is the FMF toggle code built into FPBX relies on the callerID to determine the current user, and ignores the value that you pass in. So even though you try to dial *21XXX, the actual FMF macro just ignores the XXX and tries to find the extension to toggle based on the callerID… which from an IVR of course doesn’t make any sense.
Only other way I can think of to do it would be to write a basic custom context that would do it for you. That would have the advantage of course of being able to be one destination that would work for any user since you could prompt them for their extension/VM password before letting them do it… unless somebody else has a better suggestion.