Under Section 20.19(h) of the FCC’s revised HAC rules, beginning January 15, 2009, each manufacturer and service provider subject to the HAC rules that operates a publicly-accessible website must make available on its website (1) a list of all hearing aid-compatible models currently offered, the ratings of those models, and (2) an explanation of the rating system. Each service provider must also specify on its website, based on the levels of functionality that the service provider has defined, the level that each hearing aid-compatible model falls under as well as an explanation of how the functionality of the handsets varies at the different levels.

HAC is best described when mobile phones and hearing devices (hearing aids and cochlear implants) are used together the result is buzzing, humming or whining noises that are detected by the user. Some hearing devices are more immune than others to this interference noise, and phones also vary in the amount of interference they generate.

The wireless telephone industry has developed two types of ratings for mobile phones to assist hearing device users in finding phones that may be compatible with their hearing devices. 

M-Ratings
Phones rated M3 or M4 meet FCC requirements and are likely to generate less interference to hearing devices than phones that are not labeled. M4 is the better/higher of the two ratings

T-Ratings
Phones rated T3 or T4 meet FCC requirements and are likely to be more usable with a hearing device's telecoil.

It is important to try the different features of any phone thoroughly and in different locations, using your hearing aid or cochlear implant, to determine if you hear any interfering noise. Consult with NVC or the manufacturer of the phone for information hearing aid compatibility.