Also known by other terms (T switch, telecoil T coil, telephone pickup switch, telephone induction coil, telephone induction pickup coil, etc), these describe that special feature on hearing aids which uses the principle of inductive coupling. This inductive coupling has two major possibilities:

It can be used with a "hearing aid compatible" telephone for communicating on the phone.

It can be used with assistive listening devices and loop system.

When hearing aids are set to this position, the induction coil or telecoil is activated to pick up stray electromagnetic energy from the telephone and convert it to amplified sound. Because the hearing aids' microphones are not functional during this action, the only sound being amplified is that directly from the telephone - without any outside interference.

About Telecoil Use

"Why can't telephone induction pickup be used with all hearing aids?"
Successful use of hearing aids depends on:

The hearing aids must have an induction coil and associated electronics which are strong enough to pick up and convert signal. Just because the hearing aids has an induction coil does not mean that it will work well with the telephone. The electronics and size of hearing aid usually dictate how strong the induction coil will be. Essentially, the smaller the hearing aid, the weaker the induction coil, the less chance it has of being effective, and less space for it's inclusion.
The telephone (or other device used instead) must produce a strong magnetic field. For many years telephones, through poor design,produced strong magnetic fields that hearing aids could pick up on. However, improvements in telephone receiver designs which made them more efficient, durable and less expensive to manufacture, also reduced or eliminated these strong magnetic fields. Still, telephones vary in their ability to produce a good magnetic field which hearing aid can use effectively. Sometimes the combination of a weak induction coil in the hearing aid and poor quality hearing-aid-compatible telephone produces very poor results. It is suggested that when buying a "hearing-aid-compatible" phone, that it be tried out with your hearing aids first.

The location of the induction coil in the hearing aid must be properly oriented for best reception. Because of the design of a hearing aid, the induction coil cannot always be placed in the most effective position. Move the telephone receiver around (with different angles) the hearing aid to locate the point where the induction coil in the hearing aid is very close to the magnetic field of the telephone receiver.
Successful use depends also on the environment in which the "T" switch is used. Interfering sources such as power lines, transformers, and flourecent lights can make the use of this feature impossible.

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Hearing aids telephone pickup function

About Telecoil Use