This Wired How-To Wiki article on tapping phone lines is a good primer on what actually happens when someone puts a physical tap on your line. Of course, there are lots of invisible ways to virtually tap your line: in the US, the Federal CALEA statute mandates that phone-switches have tapping back-doors that only cops are supposed to have the passwords for (yeah, right), and the digital PBX in your office is just as likely to have a vulnerability as the PC on your desk.
The Tap:
With an access point in mind, you should have an idea of the necessary equipment. Using an old lineman's handset (also called a "butt set") or building a "beige box" are the best starters. In short, the lineman's handset is a tool used by repairmen to test a line for activity. It's little more than a trussed up wall phone with a small dialing pad and alligator clips for tapping directly into a line. A beige box is just the DIY, 133t cousin of the lineman's handset. Of course, if price and jail time are no concern, there are a number of other options — but for the sake of ease, we'll stick with these.
(Image: Trussell)
