I’m with Per here, in that you’re wrong.
The call waiting thing is handled by your hanging up or pressing a ‘call waiting’ button (which hangs up for a set ammount of time) signalling the telco switch, which then connects party #2… The connection is always there, the telco handles switching the right party onto the line.
The bit about the lock trace is partly correct… The idea of the high voltage wasn’t to change what your phone did, but what the switches did. Old switches locked into a call with a relay, when the voltage dropped, the relay clicked off and the switch reset that ‘port’. The idea was to force the voltage high to keep the physical connection high. Theoretically you could short the line and drain enough current to reset the switch. It’s not that it prevented you from hanging up, because the power in the phone line was being sent from the phone company, it was to prevent the switch from hanging up when your phone signalled it (by being on hook) to stop sending the extra power.
As for the OP. No, there is no way with most older phones, the handset is unpowered unless the phone is picked up, that’s just the way the circuitry worked. In a newer phone, you really don’t know what they do, being that instead of nice traceable circuits they have ICs… You can test the handset to see if it’s getting power though, but you don’t know if the speaker-phone mic is being powered (actually, in all likelihood, it is, even in unbugged phones). And they could easily make a phone that accepted incoming calls, delayed the ring just a bit, and listened for a secret signal, then upon receiving that, go into bug mode. But it’s unlikely that it’s done on a big scale because that would cause problems with long distance calls being billed when the phone ‘answered’ even if the human didn’t… people would notice this and think it was a problem, if not a bug. (of either sort.)
Argh, I hate that law. It’s a true example of outlawing something so that only outlaws have it.
They could have just encouraged encryption to be used, but they made it illegal to listen, thus ensuring that few enough people will take advantage that it’ll never really hit the news, but enough will to be able to profit from it. If it was simply encrypted, it would be hard for anyone, not just honest people.
That, and laws forbidding people to sample certain parts of the EM spectrum seem… wrong. It’s like forbidding people to see anything blue.
Not that this is your law or anything, but I just thought I’d rant a bit…