Is there a way to test phone jacks to see if they are actually "on line"? I have quite a few phone jacks to test at work the way I do it is I just grab a crappy phone and see if there's a dial tone on the line but I need something more compact that I could carry in my pocket. Would a phone cord cut in half with a resistor and a LED in series across the red and green wires work? Basically the phone jacks go to a central office where they get patch so I need to determine which ones are patch and which ones are not.