Saw this in Wikipedia (http://en.wikipedia.org/wiki/Iphone_4):
Hearing aid compatibility
GSM 3G 850/1,900 MHz M4, T4 2G 850 MHz M3, T3 2G 1,900 MHz M2, T3 CDMA M4, T4
So what do M4, T4, M2, ... mean?
Ask Different is a question and answer site for power users of Apple hardware and software. It only takes a minute to sign up.Sign up to join this community
If you see a M3, M4, T3 or T4 on the box then the phone has been designated as HAC compliant.
About the T
"T" refers to the phone's coupling ability and is intended for use with hearing aids in the telecoil mode. The higher the "T" number the more likely you will be able to use the phone with your hearing aid on the telecoil setting. A telecoil is a small device that is built into some hearing aids for use with the telephone as well as assistive listening devices. To use the telecoil, generally either the hearing aid is switched to the "T" position or a button on the hearing aid is pushed to select the telecoil program. The telecoil picks up magnetic fields generated by telephones and converts these fields into sound. Telecoils are particularly useful for telephone communication because they permit the volume control of a hearing aid to be turned up without creating feedback or "whistling," and background noise can be reduced especially when using cell phones in noisy places. That is why hearing aid phones are important. They give you the ability to control and process sound.
About the difference between M and T
An "M" rating measures compatibility with acoustic hearing aids while a "T" rating measures compatibility with induction hearing aids. So the higher the number (e.g., M3, M4 etc.) the more compatible the handset. Generally, handsets rated "M3" or "T3" by ANSI or better are considered compliant with FCC requirements. Acoustic Hearing Aids M3-rating indicates the handset has satisfied the ANSI standard M4-rating indicates the handset has exceeded the ANSI standard
Induction Hearing Aids T3-rating indicates the handset has satisfied the ANSI standard T4-rating indicates the handset has exceeded the ANSI standard