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November 04, 2007


Mike Pierce

Sure, Ooma keeps making these claims, but the truth is the following:

1. There are ways around the Calling Line ID blocking. Check out
Besides, any such blocking does not prevent the police from getting your ID for an illegal call.

2. Of course the Ooma Hub can detect other "extensions" going off hook by detecting the drop in line voltage. Most answering machines have done this for years. (In fact, mine often fails to detect an extension going off hook because of the low voltage the phone company feeds these days.) The privacy problem is about using things other than regular "phone extensions" to do the monitoring. Check out
As some one else noted, you can buy a telephone "butt set" in Home Depot that allows "high impedance" monitoring. It's the same as what I used to carry when I worked for AT&T.

I wish Ooma would come clean.


Mike, thanks for the note.

1. Yes, "CallerID blocking" is implemented at the PSTN switch that delivers the call to the called party, so it only works at a consumer level. The calling party number is still passed through the telephone network in the SS7 ISUP setup message, so any company whose equipment is interfaced to the telephone network via SS7 signaling has access to calling party info despite blocking, for example, the call center mentioned in the post you point to.

2. Yes, the circuit shown is exactly what I meant by a high impedance listening device. Actually, you don't even need an amplifier. The minimum high impedance listening configuration I've seen is a single coupling capacitor connecting a ordinary phone which is then locally powered by a 9 volt battery in series with a 620 ohm resistor (to give about 15 ma of "talk battery"). You connect the battery-resistor power source across the two leads of the phone so the phone is "live" and you connect the phone to the line via one (or two) blocking capacitors so it doesn't load the line.

In any event, the ooma story is rather lame.

MIke Pierce

Yes, I believe a simple capacitor with a phone would work. Since the point is to do it unnoticed, you would not want the "talk battery" connected.

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