Just finished hooking up 8 of these buzzers for the phone system on my model railroad. Everything is working, but I found some of them temperamental. Not sure what the problem it, but I found if I just left them hanging by their wires they would work. It could possibly be the wire connection inside the speaker, but I'm not sure.
[Ted is correct, the 5V buzzer is temperamental, we offer it for those want to use an existing 5V buzzer supply. We will be offering 12V buzzers in the near future, or you can put a note on your 5v order that you want a 12V buzzer instead -- MRCS]