Don't forget though that u can make the doorphone page instead of ring all phones. Set the rings to None, give the doorphone access to whatever page zone your using, and make it PAGE everyone when someone is at the door. Program a speeddial on all phones to call the doorphone. You can only put so many Answer DN's for the same extention. If the place is big, it won't work so great.
Not sure if you can give each door their own page sound though. I would hope you can.