A telecoil is a tiny component in a hearing aid or cochlear implant processor that allows one to connect directly, with a variety of audio technologies. The telecoil really should be called an 'audiocoil'. It is referred to as a telecoil because it was originally designed to be used with older landline telephones. Today it can connect with all personal audio devices that have an input jack. That includes your cell phone, computer, iPad, radio, etc. It will also allow you to connect wirelessly to a public address system in a large venue if a hearing loop is installed in that room. In all instances, turning the hearing aid to telecoil mode allows you to hear ONLY what is coming into the microphone or from the device it's plugged into. It does not pick up background noise in the environment, as it brings the sound direct to your ear. Some say it's like having binoculars for the ears! Because this component was originally designed for old landline telephones that were used in the 40s, some who don't understand its myriad uses consider it 'old technology'. It IS old technology that remains invaluable to hearing device users who wish to remain in the hearing mainstream. And, it is available in most hearing aids with the exception of tiny in the canal aids that don't have space for the component. If present, it must be activated by the fitter to be functional. And, the person who fits your hearing aids or cochlear processor has an obligation to tell you about it and to explain how it works. If you ask about it and they say "It's old technology you don't need." They are wrong.
Too many people, including those who sell hearing aids, think that BlueTooth technology has replaced the telecoil. In some instances it has, but not in large group/room settings. (Theaters, performing arts centers, lecture halls, worship centers, etc.) Consider this: If there were 20 hearing aid users in a lecture hall with 100 listeners, the speaker would need to have 20 BlueTooth microphones to transmit to those 20 people. With a hearing loop and telecoils, only the main microphone that transmits to both the hearing loop and the public address system is needed to transmit to all of them. No receivers needed. All one has to do is turn the telecoil on to connect.
It doubles the value of your hearing instrument.