Hearing aids are designed to amplify all sounds for the user, both desired and undesired sounds. As a result of this amplification, some hearing aid users can experience uncomfortable interference while using wireless phones. To eliminate this interference, some phones have been designed to operate with hearing aids, while others have been designed to operate with telecoil hearing aids, where the hearing aid receives only the signals from magnetic fields generated by the telecoil-compatible phone. To assist users in determining which wireless phones are hearing aid-compatible, a ratings system has been designed by the American National Standards Institute. Phones that are compatible with telecoil hearing aids receive a 'T' rating from one to four, while hearing aid-compatible (but not telecoil-compatible) phones receive an 'M' rating, also from one to four. Phones which receive a 'three' or a 'four' rating typically cause the least interference to hearing aid users. While the ratings system can be a useful tool for consumers when choosing the right wireless phone, it is by no means a guarantee that a phone will not cause interference with a particular hearing aid.