I made something like that but i didn't use the $VAR feature. On VMPro i add an option on the operator menu (example: 1234), this option was the password that the user should know to have access to DISA, connect this option to a Transfer box and type on the specific field $KEY. On Manager you have to make a system shortcode that matches the password you asked before, like this:
shortcode: 1234N
telephone number: N
in this way you give access to outside calls.
You can add another menu to ask for two password, one for the access of the seconde menu and you can add a recorded voice to ask the second password on the second menu, i made this to provide more security, DISA can be dangerous if you program wrong.