Phone tap detectors protect your telephone conversations but require several levels of ability for the many types of telephone conversation monitoring available these days.
Single or second level units, or some of the less expensive amateur level models can give false positives, or misleading information as well. If you are looking into phone tap detectors, really you should go for a professional level detector, or none at all. Nothing brings chaos into our lives faster than thinking a boss or co-worker is monitoring our phone conversations, when they are not.
There are a number of ways to monitor telephone conversations and modem/fax activity, the most common is simply recording phone conversations.
This can be done with a 'pickup coil' attached to the ear-piece, or an in-line tape with a recording output.(more info)
Another type of phone surveillance or phone wire tapping is the Direct Line Tap.(more info)
Radio taps are the 'bugs' we hear about most often.(more info)
Phone tap detectors needed to guard against all of these are few and far between. The amateur level models can give false positives, or misleading displays.
Professional level home or office phone tap detectors
are really what you are looking for if you suspect that your phone is being tapped, or just want to keep an eye on things regularly. Again, the hassle of false positives is great, so don't get the cheap ones.
So save yourself the hassle, and headaches, and get a phone tap detector which is fully functional, and of a high professional level.