This
information is for
educational purposes only, as it is
illegal to commit telephone
fraud, or any other form of
phreaking.
Here is a basic primer on tapping a telephone line. To tap a phone line, you will need a few things: A telephone (a flip-phone from Radio Shack works well), a length of phone cord (have two phone cords, one with the end cut off), and a pair of alligator clips. You might also want both a philip's head and a flathead screwdriver.
First, let's discuss what's inside a phone cord. If you were to cut the end off of a phone cord and strip the outside casing, you would find four wires (remember, this is verified in the United States only). One yellow, one black, one green, and one red. We are only concerned with the red and green wires. Strip the red and green wires down, and attach the 'gator clips to them. This will enable you to hook onto the screws inside of a telephone box.
There are two main typs of telephone boxes to tap. One is the "Customer Interface Box," usually located on the side of a house or small business. The "customer" has access to the left half of the box, which can be opened with a flathead screwdriver. Upon opening a customer box, you will find a normal phone cord plugged into a telephone jack. Unplug it. Now, plug your normal phone cord into the jack, and plug the phone cord into your phone. If you can hear a dial tone, you can now tap the line.
To tap the line, plug a telephone line splitter into the jack, and have one line (their line) going into half of the splitter, and your line going into the other. When the phone is in use, you can listen in on the conversation. A more sophisticated setup would be to set up a tape recorder system to automatically record all telephone calls.
To access most other telephone boxes ( the ones on street corners, etc), you will need a special socket wrench to open the box. Fortunately, you can just use a pair of pliers to turn the bolt on the side of the box and open the case. Once the case is open, you will see a dual row of screws. This is where your telephone cord with the 'gator clips comes in. The screw on the right is called the Ring (dating back to the days where operators had to manually connect calls, they would plug a cable into a hole, and the Ring was the outside part of the cable plug). The Ring is for the red wire. The screw on the left is called the Tip, referring to the "tip" of the cable plug in the older days of telephone systems. The Tip is for the green wire. A good way to remember this is a simple phrase: "Red-Right-Ring". Now you won't ever forget which way to attach the 'gator clips!
Attach the red wire to the right screw, and the green wire to the left screw, and you should get a dial tone. If so, you can once again make calls/tap the phone line. There are many other ways to tap telephone lines, and these ways are very basic, but this should help anyone interested in the basics of how tapping a phone line works.