Bonded ISDN lines are called a PRI (also known as a T1). A normal ISDN BRI consists of one D-channel and two B-channels. A T1 was one D-channel and 24 B-channels. You could also bond multiple T1s together; it was effectively a long-distance serial link that operated off the telephone network.

Technically you could also bond multiple BRIs together, but pretty quickly it made more sense to just provision a PRI with some of the B-channels disabled (also known as a fractional T1).