All vendors still base it on the same ranges. They dont want to do unnecessary sampling and would rather keep down the sample rate.
Its all based on Nyquists theorum, that you sample at a rate twice the frequency.
This is why TDM is based on 64k channels. Voice is sampled between the frequencies of 0-4000 Hz. 4000 Hz *2 = 8000 samples and 8 bits per sample = 64 kbps per channel.