The Otis elevator man was on site when I visited, and he told me that as long as the front desk is monitored 24 hours per day, this is OK. The building has a standby generator and the phone system has a big rack full of car batteries backing it up. (This is an old phone system about the size of a washing machine.)

Is there such a thing as an extension circuit that detects when a phone goes off hook and automatically connects it to another extension?