Digital signal 0

Note 1: The DS0 rate forms the basis for the North American digital multiplex transmission hierarchy.

Note 2: The DS0 rate may support twenty 2.4-kb/s channels, or ten 4.8-kb/s channels, or five 9.67-kb/s channels, or one 56-kb/s channel, or one 64-kb/s clear channel.

Source: from Federal Standard 1037C

To carry a typical phone call, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation. Multiple DS0's are multiplexed together on higher capacity circuits. 24 DS0's make a DS1 signal, which when carried on copper is the well-known, T-carrier system, T1 (the European equivalent is an E1, containing 32 64 kbit/s channels).