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, which means you can just call it anytime you want to eavesdrop. Better yet, you can set it so that anytime there’s a sound above 45dB within 10m, it calls you!
– Use with any mobile SIM card (We recomend Verizon Wireless): Insert the SIM card; conceal the device where it can’t be seen; dial the SIM card phone number from your phone; now you can listen in!
Again, not sure how legal something like this is in your jurisdictions, so use wisely. It’s $60.