no, it was Strangers. the one about the, well, strangers, meeting up in like Las Vegas or something, all of them having gone through an horrific experience (i will not spoil the book- haven’t read it in almost a decade, but i remember liking the book a lot- read it!).
anyway, the device is an infinity transmitter. old versions of the infinity transmitter would make the callee’s phone busy, but not so with the newer models.
you would hook the transmitter up to a phone line, phone the number, and voila! the microphone in the hand set of the callee would turn on. as long as the phone was plugged into a jack, the microphone could be turned on. and the caller could listen to his heart’s content.
this machine would not work on cell phones, and i’m not sure about newer phone networks and newer phones. most certainly would not work for cell phones (although i’ll bet there’s an ingenius bit of programming which can turn on the microphone from a distance).
that’s all i can remember,
p.s.- also, strangers talked about a laser-based microphone, which is also a real device. point the laser at a window, and sound waves bouncing off the window make the laser spot vibrate slightly. a camera on the microphone picks up the vibrations of the laser, and translates those movements into sound. so even if the infinity transmitter doesn’t work anymore, you can still be surveilled.