[bradleyland]

Not to be argumentative, because I really appreciated that link :) but POTS stands for "plain old telephone service". It's still in use all over the place today.

The distinction is in the type of phone attached to the POTS line. It looks like it requires a combination of a carbon mic and an old, non-integrated-circuit switchhook. Something like you'd find in an old Western Electric 2500 [1].

A carbon mic has some pretty unique properties. Base output is very high, such that output is easily detected at a distance without amplification, and they're very low impedance. Even slightly newer telephone designs would use an electret style microphone. The most basic electret circuits require a capacitor, which is noted to defeat the frequency flooding attack.

1 - http://en.wikipedia.org/wiki/Model_500_telephone#Model_2500