Detect current on intercom
Play safe and avoid your arduino being fryed by something you haven't noticed like: -


Maybe there are big spikes on the wire that your multimeter cannot detect
What appears to be ground on the intercom may be several volts different to GND on arduino
Worse than (2), where does the intercom get its power from - maybe it's OK maybe it's not - what sort of isolation to AC mains does it have - do you know? I can nearly smell burning just thinking about it!!


I'd use an opto coupler to detect the voltage you want. At least if you fry the opto you can just go and buy another and not get raging mad like I would if I'd fryed the main CPU due to my own carelessness.