Samsung SCH-R640 User Manual page 218

The ratings are not guarantees. Results will vary depending on the
user's hearing device and hearing loss. If your hearing device
happens to be vulnerable to interference, you may not be able to use
a rated mobile device successfully. Trying out the mobile device with
your hearing device is the best way to evaluate it for your personal
needs.
M-Ratings: Mobile devices rated M3 or M4 meet FCC requirements
and are likely to generate less interference to hearing devices than
mobile devices that are not labeled. M4 is the better/higher of the two
ratings.
T-Ratings: Mobile devices rated T3 or T4 meet FCC requirements and
are likely to generate less interference to hearing devices than mobile
devices that are not labeled. T4 is the better/higher of the two ratings.
Hearing devices may also be rated. Your hearing device manufacturer
or hearing health professional may help you find this rating. Higher
ratings mean that the hearing device is relatively immune to
interference noise. The hearing aid and wireless mobile device rating
values are then added together.
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