What is the frequncy range of digital signal?

Telephone bandpass is 300Hz to 3000Hz. This is adequate for a recognizable and understandable voice, however the lack of high frequencies makes some people sound different on the phone.

Local calls often have a slightly wider bandpass, but long distance calls are sharply filtered to cutoff at exactly 3000Hz, to avoid spillover into adjacent channels during the process of frequency division multiplexing so that many voice connections may be sent on one line (e.g. twisted pair, microwave link, optical fiber) at the same time.