Hearing-impaired cell phones work either by amplifying the sound from the earpiece or having compatibility with a hearing aid. Amplified hearing-impaired cell phones make hearing sounds easier, while hearing aid-compatible cell phones prevent audio interference between the phone and the hearing aid.
An amplified cell phone works by increasing the volume of the sound coming from the cell phone's speaker. Amplified phones increase the volume of outgoing sound to levels up to 50 decibels over a typical phone's volume.
Hearing aid-compatible cell phones are not amplified, but they use specific frequencies that do not interfere with hearing aids. Hearing aid-compatible cell phones in the United States use two standards, the microphone and telecoil ratings, that describe how well they work with hearing aids. The telecoil rating does not have a large impact on compatibility, but the microphone rating does.
A cell phone's microphone rating, called the M rating, is a number from M1 to M4. The hearing aid has its own M rating on the same scale. Adding the M ratings of the hearing aid and cell phone together produces a number that describes how compatible the two devices are. An M8 rating indicates the best possible compatibility, while any M rating less than M5 is unacceptable.