An infinity transmitter (also known as a harmonica bug) is a surveillance device used to covertly monitor conversation in a room through a telephone line.
Its name derives from the fact that, by using a telephone line as a transmitter, it can work at an infinite distance, unlike other bugging devices that have only a finite signal range.
[1] The alternative name 'harmonica bug' refers to the fact that such devices were originally activated using the tone produced by a harmonica.
[2] As modern telephone lines no longer establish a voice path until the call is answered a variant of this now exists that uses CND, or caller ID.
In much the same manner a cellphone can be configured for silence on ring and auto answer and hidden, frequently placed inside something that has power available to maintain the battery.