I am familuar with setting up a GDM in Unity Express but in Unity Connection 7.0 I can't really find any info on it. Is it called GDM? The closest thing I can find is Alternate Extensions!! It that the equivelant?
You will need to create a mailbox for the GDM and give it an extension that your phones will use. Create a shared line on all the phones that will need to access this mailbox.
This way the MWI will light on the phones. There are other ways of accomplishing the same thing, but I find this is the easiest for the end user.