There will be way better answers here from those in the know, but here is a quick and dirty one.
At our plant we have it set up so that when someone calls in and gets that persons Voicemail, there is the option to hit the "6" key on the keypad to have that person paged. If that is set to Yes then that function will work, paging through the speakers that person has a call waiting.
I think thats right but the other guys will let you know for sure.
"Silence is golden, duct tape is silver...