[zengargoyle]

Back in the late 1980s when I was a teenager I found plans and built this little circuit with a LED that would light up if someone else in the house was trying to listen in on my phone calls. More than one phone off the hook... LED lights up.

[anonymousiam]

Good project for a comparator chip! Just set the trigger threshold slightly below the off-hook voltage for your extension and it shows when another local phone is also off-hook. Only works locally though, and will not let you know if your phone is tapped, or if a nosy operator or phone company technician is listening.

A long time ago, I helped a co-worker tap his own phone because he suspected his wife of cheating. I gave him a device that would connect to a phone line, activate a cassette recorder when any phone went off-hook, and record the audio. He ended up getting evidence for a divorce shortly afterward.

The "butt set" phones that telco field techs have will let you connect to a line and monitor a call without affecting the voltage, so the comparitor/LED doesn't ensure privacy beyond your family extensions. I still have an old one (with a dial). It's also useful to find the right pair with a flicker test set. I still have a few of those too.

Sometimes a field tech needs to access the CO and will find an unused line on a nearby demark. Long after my phreaking days had passed, a friend of mine had connected a DTMF decoder to his phone line (AC coupled, so it was undetectable). One day he looked at his logs and found that he had recorded a session between a field tech and the CO. The phone number and access codes were his for the taking! Fortunately for the phone company, my friend is mostly a trustworthy law-abiding guy.