Comment by anonymousiam | Hacker News Reader

Comment by anonymousiam | Hacker News Reader

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POTS phone wires typically have 45 volts DC across them when the phone is on-hook. When off-hook, the voltage goes down to just a few volts. When the phone is ringing, you'll get about 90 volts AC, so if you have high skin resistance and you were holding the wires, you'll get a jolt.

Where the article first refers to "off-hook" voltage, it should be "on-hook" voltage. When the phone is on-hook, it presents (approximately) infinity ohms of resistance to the line. The reactive load will obviously be a lower impedance because of the capacitor-coupled ring circuit.

On some of the pre-ESS (GTE) switches I phreaked on as a kid, the polarity of the DC voltage going to the calling phone would reverse when the phone being called was answered. You could put a diode in the right direction on the calling phone and it could only ever receive calls thereafter. If an outgoing call were placed, it would dial and ring, but would disconnect (hang-up) when the remote side answered. You could also put an LED across the same diode (oppositely polarized) and it would light up to show you when the remote phone was answered.

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.

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.

> POTS phone wires typically have 45 volts DC across them when the phone is on-hook.

Don’t you mean -48 for the US phone system?

48VDC is specified, but in my experience, it varies. The bottom of the article says the same:

On-hook voltage: 40-50 V DC

Pretty sure from memory AT&T spec’d negative voltage to reduce corrosion.

Ah. I did not see that you meant to emphasize the negative vs. positive voltage. It's true that it's negative, but given that phones are polarity independent, it doesn't really matter. I was expressing the voltage as an absolute value.

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