Let’s not get into the legality of this device. But if you have a need to listen in on conversations for whatever reason, the Spy Bug Audio Surveillance gadget pictured above should do the trick. About as small as a pack of matches, it accepts a SIM card,
Room Spy Bug Audio Surveillance Listening Device
Voice Recorders Dictaphones: Mini Spy Voice Recorder Sound Activated Listening Device 286H Recording Capacity -> BUY IT NOW ONLY: $96.97 on eBay!
Voice Recorders Dictaphones: Mini Spy Audio Recorder Voice Activated Listening Device 286 Hours Record Life -> BUY IT NOW ONLY: $99.79 on eBay!