The real analog copper lines were kind of limited to approx 28K - more or less the nyquist limit. However, the lines at the time were increasingly replaced with digital 64Kbit lines that sampled the analog tone. So, the 56k standard aligned itself to the actual sample times, and that allowed it to reach a 56k bps rate (some time/error tolerance still eats away at your bandwidth)

If you never got more than 24-28k, you likely still had an analog line.