One thing you can see in How Telephones Work is that the phone system is extremely simple. It is possible and relatively easy for someone else to tap into your line, especially if you live in a dorm or an apartment building. If someone has access to the wire, they can connect a phone to it and use your line. This is pretty rare because, for one thing, the penalty for doing it is stiff. It is also relatively easy to detect and, in general, phone wires are not that easy to get to because they are hidden inside walls. However, it is not unheard of.
There are two possibilities:
It is fairly easy to catch someone doing this.
One thing you can do is get a tape recorder
for phone calls. Radio
Shack, for example, sells them. The recorder is completely automatic and starts recording as soon as anyone picks up any extension (which would include an extension tapped into your line). It captures both sides of the conversation. You could listen to the tape and probably get a good idea of who has tapped in.
- The perpetrator can create a permanent splice into
your wire, in which case you would be able to see the splice if you can
trace the entire length of the wire.
- The perpetrator can clip in
intermittently at an junction box. This is what a telephone repair person does when testing your line. In
that case you would have to catch the person in the act because there is no physical evidence.
The other alternative is to buy a phone that lights a light whenever any extension is in use. Then you would know when another person is making a call, and you could pick up your phone and listen in. The tape recorder option is better because it produces physical evidence.
Hooking into a phone line like this is, as you would expect, a federal offense. Once you have evidence that it has occurred, turning it over to the phone company or police department for prosecution is the correct path to follow.