What kinds of phones support detecting DTMF tone duration?

I’m a little confused by this answer (from http://www.voip-info.org/wiki/view/Asterisk+DTMF):

Living in the US, what kind of phones would I be able to do this on (what about cell phones?) My hope is to have a DTMF tone-duration detection system that works on most phones.

Thanks in advance for the help!