Hearing aids have not in the past always worked well with cellular phones, because of electronic interference between the 2 devices that caused static, whistling or screeching noises, or lost words. Technology enhancements along with new regulations have mostly eliminated this problem. Nowadays cell phone – hearing aid compatibility is not the problem it used to be. The labeling requirements mandated by the new government regulations make it easy to find a mobile phone that is compatible with your hearing aid.
Understanding the rating system requires a bit of knowledge about the modes that hearing aids can operate in.
There is an M mode (which stands for microphone) and a T mode (which stands for telecoil). In M mode, the hearing aid uses the internal microphone to detect sounds and amplify them. When the hearing aid is in T mode, instead of the microphone it uses its built-in telecoil to directly pick up conversations from inside the phone, in the form of electromagnetic signals. Roughly 60 percent of all mobile phones sold in the US have a telecoil (T) mode.
The rating system for these two modes of hearing aid operation uses a scale that ranges from the lowest sensitivity (1) to the highest sensitivity (4). No mobile phone or cordless handset sold in the United States can be sold as hearing aid compatible (HAC) unless it has a rating of at least M3 or T3.
Hearing aids themselves also carry M and T ratings to indicate their sensitivity and ability to block interference in each mode. If you know the M and T ratings for your hearing aid, to determine its compatibility with any mobile phone, just add the two sets of ratings together. A sum of 6 or more makes a solid pairing. That hearing aid and cell phone combination should work well for you. If the combined rating is 5, this combination is considered normal and suitable for most regular phone use. A combined rating of 4 is considered usable for brief calls, but may not be suitable for extended phone use.
If you are shopping for a mobile phone online, you can usually use this combined rating to determine how compatible the phone you are interested in buying will be with your hearing aid. In the end, nothing beats a real world test so you may want to wear your hearing aid to the mobile phone shop and test out a few different phone in real conditions.