The calling group would be programmed via the phone system. Then the VM system would be set to "transfer to an extension" when 0 is pressed.

If you have a computer with WinSPM it's just a matter of doing a print out of the calling groups.

If you don't have WinSPM, you can find the same info via system programming on the programming phone under group calling members.

The other way would be to call each calling group number until it rang the 4 phones in question.
Odds are, it would be 770-779