ISDN Basic Rate is an example of narrowband digital telephone service. Broadband circuits carry anywhere from 23 to several hundred telephone conversations per circuit. T1, E1, ISDN Primary Rate, and T3 are examples of broadband digital circuits. For all of these telephone circuits, the basic idea is the same. The principal advantage of going digital is that there is little corruption of data between the origin and destination.
Bits go in, and the same bits come out on the receiving end.
Of course, if the circuit is not configured correctly or is malfunctioning, data will be lost, but when it is operating normally, the audio reproduction is clearer than on an analog line where noise and attenuation will creep into a phone call. While the marginal increase in call quality is a nice bonus, there are several unrelated reasons for going digital. All telephone calls can be carried over a single circuit rather than many single channel circuits. On analog circuits, the telephone company will typically only provide one or the other, but not both. With digital, you can get both Caller ID and DNIS from many telephone companies.