Remember ISDN? It was the 1980s digital connect methodology that was going to bring us everything digitally. In truth, ISDN (a.k.a. BRI) is the “little brother” of T-1 (a.k.a. PRI). PRI is actually the signaling scheme, but the relationship is accurate. Whereas a T-1 is 24 x 64kbps voice channels and signaling an ISDN circuit can be 2 x 64 kbps channels (Bearer or B channels) and signalling (a D channel).
In Europe ISDN is quite common, having been a valid alternative to analog POTS lines for many years. In the US it never caught on the same way. It’s common in certain vertical market niches. For example, just about every significant recording studio used ISDN as a means of transferring audio across country in real-time. It’s a channel with assured bandwidth. The two B channels could be bonded yielding a solid 128 kpbs. Throughout the 1990s voice-over talent working from home studios (Hi Alison!) almost always used this sort of a setup.