Hearing instruments can amplify sound either through the microphone, through the telecoil or even both simultaneously.
A telecoil is a tiny coil of wire fitted into hearing devices, enabling the wearer access to an audio induction loop. The loop carries amplified sound such as speech or music and transmits it in the form of an electromagnetic field to the telecoil in your hearing device.
The telecoil program can be accessed on the hearing instrument either by a program button, switch or remote control. When you are in this program, the microphone is usually switched off, reducing interference from background noise, making it easier for you to hear what is being transmitted.
Not all hearing instruments are fitted with a telecoil. Most behind-the-ear style aids have a telecoil, but some small in-the-ear styles, do not have the room to fit a telecoil.