The name T-coil stands for Telephone-coil. Talking over the phone
was its original purpose. Nowadays, T-coil is used on many other cases,
not just for solving phone talking issues.
When you switch to T-coil mode in your hearing aid, you choose to use a different ‘input’ to the hearing aid, the T-coil, instead of the microphone.
One of the biggest advantages of the T-coil is that it is generic and widely spread on many different hearing aids. It is not unique to a certain company. Given that a hearing aid has a T-coil option, you can use many other options as the source, not just the phone. For instance, a neck-loop, or hearing loops they use on public places. All of these instruments send magnetic signals that the T-coil can receive inside the hearing aid and translate it into audio sounds.
This is an example of one usage out of T-coil. It illustrates a cord that connects your phone, for instance, to your hearing aid. It is doing so by using a shilluete as the magnetic inductor placed next to your hearing aid.
The best way to explain what T-coil is, is to think of it as a microphone for electromagnetic signals. Any device that transmits these signals can connect to the T-coil inside your hearing aid.
Another way to think of it:
The microphone in your hearing aid reacts to sound waves and turns it into electric signals. T-coil, on the other hand, reacts to a magnetic field and turns it into an electric signal inside your hearing aid.
On T-coil’s original use, it gets the magnetic signal from a phone. It bypasses the phone’s speaker and the hearing aid’s microphone. Instead, the phone’s magnetic signal is transmitted to the T-coil.
This way, you can save lots of noises, and the overall hearing of the phone is improved.
I write this for the sake of hearing aids users that find themselves confused by all the wireless terminology and misunderstandings. Therefore, I try to keep things as simple as possible and I simplify things to make them clearer. In addition, the discussion here is in the context of hearing aids and hearing aids’ users. I don’t try to cover the entire wireless world here, and therefore, all the data is observed from the angle of a hearing aid user and for his/her benefit.