I don't know about this system, but here are some ideas, not all work on 
all systems.
I don't know if it can ring multiple extensions, distinctive ring, have 
a speaker hooked up i.e. in the hall, garage, and outside, or perhaps 
use call pickup.

I am almost certain that the best solution for a door phone might be to 
be able to ring multiple extensions. If they are outside anyone can let 
them in. Or ring a dummy phone (ringer only) in the hall. Then anyone 
can use call pickup to answer the door, or they can just walk to the 
door like it was a doorbell.

For an intercom, it might be nice to page "overhead" and have them call 
extension 101. Unless you use a cordless and always have the phone at 
your side.