You are buying Avaya Nortel Norstar NT8B79FD Door Phone Stainless Steel

It allows 2 way conversations from any inside Norstar Phone to the Norstar Doorphone

Works on Norstar 824 Modular KSU, Compact, Norstar Compact CICS KSU, Norstar Modular MICS, and the Nortel BCM Phone System

You can get an optional door opening controller (DOC), controlled by pressing softkey for additional $125 refurbished, like New