Hearing aids do not always function well with wireless handsets. Hearing aids operate by using a microphone to pick up sound waves, converting the sound waves into electrical signals to be amplified. Distortion or amplification of unwanted sound (noise) often occurs.

The FCC's hearing aid compatibility requirements address hearing aids that operate in either of two modes – acoustic coupling ("M" rating) or inductive coupling ("T" rating). Hearing aids operating in acoustic coupling mode receive through a microphone and then amplify all sounds surrounding the user, including both desired sounds, such as a telephone's audio signal, and unwanted ambient noise. Hearing aids operating in inductive coupling mode turn off the microphone to avoid amplifying unwanted ambient noise, instead using a telecoil to receive only audio signal-based magnetic fields generated by inductive coupling-capable telephones. The FCC's "M" and "T" ratings indicate whether a handset can be expected to function well with a hearing aid and are generally marked clearly on the handset packaging. The number associated with the "M" or "T" rating indicates the HAC handset's level of immunity to electromagnetic noise, with 4 having the best immunity.

The "M" or "T" rating does not guarantee that the handset will function without distortion or noise, so ETC recommends that you test the handset before purchasing.