There are different kinds of ISDN connections of varying bandwidth:
DS0 = 1 channel PCM at 64 kbps
T-1 or DS1 = 24 channels PCM at 1.54 Mbps
T-1C or DS1C = 48 channels PCM at 3.15 Mbps
T-2 or DS2 = 96 channels PCM at 6.31 Mbps
T-3 or DS3 = 672 channels PCM at 44.37 Mbps
T-4 or DS4 = 4032 channels PCM at 274.1 Mbps
Each channel here is equivalent to one voice channel. DS0 is the lowest level of the circuit. T-1C, T-2 and T-4 are rarely used,
except maybe for T-2 over microwave links.
A Basic Rate Interface (BRI) is two 64K "bearer" channels and a single "delta" channel ("2B+D"). A Primary Rate Interface
(PRI) in North America and Japan consists of 24 channels, usually 23 B + 1 D channel with the same physical interface as T1.
Elsewhere the PRI usually has 30 B + 1 D channel and an E1 interface.
A Terminal Adaptor (TA) can be used to connect ISDN channels to existing interfaces such as RS-232 and V.35.
Different services may be requested by specifying different values in the "Bearer Capability" field in the call setup message.
One ISDN service is "telephony" (i.e. voice), which can be provided using less than the full 64 kbps bandwidth (64 kbps
would provide for 8192 eight-bit samples per second) but will require the same special processing or bit diddling as ordinary
PSTN calls. Data calls have a Bearer Capability of "64 kbps unrestricted".
ISDN is offered by local telephone companies, but most readily in Australia, France, Japan and Singapore, with the UK somewhat behind and availability in the USA rather spotty.
BTW: ISDN deployment in Germany is quite impressive, although (or perhaps, because) they use a specifically
German signalling specification, called 1.TR.6. The French Numeris also uses a non-standard protocol (called VN4; the 4th
version), but the popularity of ISDN in France is probably lower than in Germany, given the ludicrous pricing. There is also a
specifically-Belgian V1 experimental system. The whole of Europe is now phasing in Euro-ISDN.