Wearable audio surveillance devices look like regular things around us. They enjoy an edge over video surveillance tools as you can just leave them at the place of interest and recording will take place by itself.
You do not have to wear listening or recording bugs to operate them for best results.
For example, you can always forget your pen in the office of your boss and come back to take it later. The best feature about these equipments is that they are extremely tiny to be recognized from a distance (5mm or less diameter) and can be easily hidden in a piece of chewing gum under the table or in a toy bug at some unsusceptible place. Being so small and hidden within something, these mini spy devices are strong enough to capture voices from a long distance effectively.
Phone Bugs are the most common audio surveillance devices that are hidden inside or around the phone of the suspect. These phone bugs may look like regular objects such as clock, tape recorder, toy etc. The advanced phone bugs function only when the phone is picked and someone starts talking, resulting into only recorded voices.
There is equipment available specifically for listening to mobile phone conversations, these high frequency receivers are usually only available to police and army personnel.