EXPLANATION
T1 connections have 24 channels in total.
Typically, for T1 ISDN
(CCS) protocols, 23 channels are used as B-channels and 1 channel is
used as a D-channel. For this reason, the frame structure of a T1 PRI
connection is said to be 23B+1D where the D-channel is located on
timeslot 23 (channels numbered from 0 to 23) and the remaining channels
are used as B-channels. Hence;
If you had an E1 PRI trunk, you would have 30 B channels and 2 D channels.
SOURCE
https://en.wikipedia.org/wiki/Primary_Rate_Interface