Nortel BST Brass Door Phone NTAB9910E6 Enables two-way conversation between Norstar/BCM phone and Doorphone Works on Norstar 824 Modular KSU, Compact, Norstar Compact CICS KSU, Norstar Modular MICS, and the Nortel BCM Phone System Has optional door opening controller (DOC), controlled by pressing softkey Up to four Doorphones can be accomodated on one Norstar system