Just as an FYI ... from a 27+ year telco employee...
The government and private interests have been doing this since years ago with the old AT&T home telephones. I suppose they changed the method since most home phones are electronic now. I suppose they can still do it on some phones.
I had a sales guy demonstrate it for me at The Spy Shop.
Make a call, person answers, you say nothing (or play wrong number), when they hang the phone up the microphone stays live. Not sure how they did it, sent a signal to the phone. The device that made this possible sold for $800 and was a big seller to private investigators, bail bonds people, etc.
This was around 1985.
He says bail enforcement agents would call a home asking about a bail jumper, have a conversation with a relative and then listen to what they said after they thought the phone was hung up. Often they would mention where the person was hiding.
Scary huh?
P.S.
http://www.2p-spyphone.com/F-A-Q.asp