Note that some buzzer systems use low voltage AC whilst others are DC but only low current so will not be able to drive a relay.
What I have also seen (in some older buildings) is a bell system that runs on 12V AC but the transformer was an auto-transformer meaning no isolation from the mains.
Your best bet is to use an opto-coupler which will provide a solution to all of the above.
Enable the pull-up resistor on the input pin of the Arduino that the opto connects to.
