Every door intercom system that I have installed on a Norstar was designed to be hooked up as an analog station.
They are run through an ATA and then the ATA extension is hotlined to a front desk phone that way when they go off hook, they immediately ring the operator.
They also make dial-pad door phones that hook up the same way. These aren't hotlined though. The visitor dials an extension or 0 for the operator.
Finally, I have seen door phones (but never installed one) that hook up through a trunk port. They are programmable. You program them to dial a number that rings in on a reception phone. When the receptionist answers, they can press a number on the dialpad that causes a contact to close. You can wire these contacts up to a magnetic door lock or an automatic gate to allow the receptionist the ability to open the door for the visitor.