[Cultural Jihad]

http://www.howstuffworks.com/static/question250.htm

[eno_]

Good call on the "modem protector," which you could use to detect fraud, as in the article you pointed to, but the poster (as ignorant as they seem to be) is worried about an eavesdropper. As long as one phone is off hook, an off hook indicator indicates the same thing. An ammeter would, however, register different readings depending on the number of phones off hook. Capacitance would probably change, too. What you want is the ability to detect the second phone going off hook, which would change both the impedence of (which is why you get a different sound quality even if an evesdropper has muted their mouthpiece) and current in the circuit.