Trace live wire without an outlet
Trial and error flipping breaker handles isn't so bad especially if you have an assistant or can attach something that makes noise (a fan, a radio, etc) so that you can hear at a distance when you find the correct breaker. Honestly it's the quickest thing you could do.
Some of us might have been known to wire 120 v devices while the circuit is live. While of course I couldn't recommend that a person should do such a reckless thing.. if a person did choose to wear gloves and work carefully, it wouldn't be impossible to attach a receptacle to those bare wire ends so that the plug-in tracer could be used (or a plug-in noise maker, for that matter). Were a person to attempt this, it might be most safely accomplished if the receptacle is prepared with some &quot;pig tails&quot; (wire leads 6 inches long or so), wrapped in electrical tape to insulate the bare terminals, and then the pig tails could be attached to the live 120 v leads with wire nuts.
If the old-school intercom is still at least somewhat functional you might be able to set it up in a way that it makes noise. Some had a built-in AM/FM radio, or an audio input jack, or maybe you can just turn up its volume and listen for the background crackly static sound to go quiet while you're going through the breakers.