M1 and M2 are the new FCC ratings for cell phones and indicate compatibility with hearing aids in their microphone settings (M2 is better than M1). Parallel to this is the T1 and T2 rating, which indicate compatablity with hearing aids in their t-switch setting (a setting on some hearing aids that allows them to get their signial directly off the speaker core, not the sound vibrations). All land line phones should be rated at least M1/T1 by law....but many do not have enough amplification for the hard of hearing...which is not part of the rating...just "compatibility". Ask client if they use a t-switch...if so get a phone with a heavy hand set...big magnet on speaker core...or has extra db amplification.