How did you get the number to passthrough? Was it a CoS option somewhere?
If you add an external number to the Controller (MCD, ICP, MiVB....I can't get my head around what it's actually called...) in the Telephone Directory form using Alpha Tagging (ie, something like "0~123987654"), then when you ring from that external number(ie 123987654), the MiCollab will display the name in the Call Window.
When it's forwarded to VM and gets emailed though, the name doesn't come through.