Good observation! They merely removed the receiver set on the right-hand side of it (they didn't even bother to repaint it)!
BTW, on Mike Matthew's office desk is another panel of telephone-switchboard-type lights. He pushes those buttons every now and again before or after talking on the intercom as if it's a master intercom system for the complex. I think Producer Irwin Allen liked them because they looked functional and made a loud 'kick' and also lite-up when you pushed them, so they made for a great 'stage effect'.
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