Also called butt-in sets. I don't do a lot of telco work, but it's a lifesaver if your working in a phoneless server room and need to dial out or call someone. With a bit of phone savvy, you can probably find a line you can "borrow".

These are basically ruggedized DTMF phones typically used on POTS lines with tough leads that go to a alligator clips instead of a RJ-11 plug. The clips are designed to clamp onto wires with or without the rubber insulation (they use little pins for piercing insulation w/o really damaging the wire). They can also jack right into a 110-style block (the thing that converts telco wiring into something you can plug into your phone system). If you have a tone and probe set, you can plug the inductive amplifier into the phone for either quiet probing or so you can hear quiet tones in a loud room. With two sets and a nine volt battery, you can talk with another set across some distance of wire, like a field telephone. Very useful.

Most butt sets have a monitor/talk switch. In talk the phone acts like a normal phone that is off hook, you can speak, listen and dial. Switching to monitor will hang up, no dialing or talking but the phone will ring in this mode and you can still hear. Hence, if someone is on the line and your are in monitor, you can hear both parties on the line but they won't know the wiser: ala the name "butt-in set". Of course this is illegal, and you could be given away if you fumble the wires, make a crappy tap or they have some detection hardware.

Another common feature is a polarity light which helps you make sure the two wires of a phone line ("ring" and "tip") are the right way around. However the set (and most phones, I think) will work no matter which way the wires are headed.

I've only owned two types of butt set: great and crap.

A great set is the Harris TS22A. It runs off a nine volt battery, has a tough, metal belt hook and awesome orange finish. The 22A has features like two-way speakerphone, speed dial and other stuff I can't even remember. It is a real battery miser, I've only changed it once for the sake of neglect. You can drop this phone through a ceiling tile, watch it bounce down every rung of your ladder and crash onto the floor, then merely dust it off and get back to work. After fixing the tile, oops.
This unit cost about $325 when I got it, probably $290 or so now. I only paid $100 tho, as a telco friend stole it from work and sold it to me brand new in the box.

A crappy set is the Model 390, by Progressive Electronics. It is powered by a proprietary battery (the phone must be sent back to Progressive for battery swapping), has a not-bad cord, plastic belt hook and is generally not built as well. I bought my first one for only $80 (they're really $160, the vendor owed me a favor) and within a few months it wouldn't dial. It just stopped making tones. The vendor said the battery should still be good, and that he has lots of these phones with that problem. (great....) He swapped it for a new one and within another few months it stopped with the same problem. I haven't performed an autopsy to find out if the battery is at fault, but I don't really care. I just use my Harris.

There are special breeds of these phones that can talk on and troubleshoot specific PBX systems, ADSL lines, and (I'd imagine) ISDN. I've only gotten to see these pricey monsters once or twice though, no real experience.

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