Hearing aids have not in the past always worked well with cellular phones, because of electronic interference between the 2 devices that triggered static, whizzing or screeching noises, or dropped words. New government regulations, together with significant improvements in both cell phone and hearing aid technology, have made this incompatibility uncommon. 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). When your hearing aid is in M mode, it uses the built-in microphone to pick up audible sounds from around you and amplify them to make them easier for you to hear. In T mode, the hearing aid uses telecoil technology instead. The hearing aid is able to pick up the electromagnetic signals from inside the phone directly. Currently, approximately 60% of hearing aids sold in the US have a telecoil or 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). To be sold in the United States as hearing aid compatible (HAC), a mobile phone or cordless handset must have 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. To determine the compatibility between your hearing aid and a mobile phone you are considering, just add the M and T ratings together; add the M rating of the hearing aid to the M rating of the phone and add the T rating of the hearing aid to the T rating of the phone. A combined rating of 6 or more is considered excellent, a hearing aid/phone combination that would provide highly usable, interference-free performance. A sum of 5 is considered normal and should work fine for typical mobile phone users. If the combined rating is 4, this is thought of as acceptable but not very usable if you make a lot of extended phone calls.
This combined rating system makes it easy to shop for a mobile phone online, because it easily allows you to determine how compatible it 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 cell phone shop and test out a few different phone in real conditions.