The problem with the line powered devices is they won't retrieve a call off hold. Most of the loop current is going through the hold bridge in the line card and there just isn't enough to work a relay coil. It's fine for getting a lamp so no one grabs the line or stopping the ringing when a call is answered, but they just will not cancel the hold. The melco devices, and by extension, my devices, will. They (in theory) will work on as little of 1 ma of loop current. It's actually looking for the 1.2 volt drop across a pair of diodes in series. That 1.2 volts lights the led in the opto and that fires the two transistors that pull the A lead to ground. It's devilishly simple. And for us nutcase collectors, really darned handy. If I answer a call on my 1A2 set on my kitchen counter, and then need to run into my workshop, I can grab the call on the partner phone out there no problem. When I was using a line powered loop current relay, I couldn't do that. It wouldn't fire when a call was on hold and I'd just hear hold music instead of the caller. Someone was asking me about creating a setup similar to mine with a 1A2 in conjunction with a Merlin system in his case. I figured it should be pretty easy to replicate the Melco, since the sandman relay I had just wasn't cutting it. That's what started this whole misadventure.
Now I'm going to have to look and see if my fax even has pins 2 and 5, let alone be wired for a leads....
