Bandwith definition

Yes,

Also a T1 line = 24 each 64 kbs digital channel slots when used as  certain types of intertrunking or terminating a digital channel bank. This is the US standard, will be 32 channel slots by European standards. The T1 line is 1.544mbs no matter how it is used.
But when a T1 line is used as Digital Concentrator trunking lines, such as the ADPCM ( Adaptive Differential Pulse Code Modulation Concentrators ) a channel slot measurement becomes insignificant.
Also I believe the ISDN contains two (2) each 64 kbs circuits and one  (1) 8 kb data circuit.