As someone that worked at a dial up ISP and had an ISDN connection at home, modem bonding was really fascinating. At the ISP side, these were all T1 lines, so every line was technically 64k capable. The analog 56k modems rarely made a true 56k connection. However, there were some customers that lived one telephone pole away from the CO, and we would see them connected at 57600. One close customer would show at some oddball speed like 58k or 59k (though we don't know what they actually got at their end).

But for ISDN, each channel was the full 64k and that would bond two channels to 128k. I had a cisco modem that would let me use either channel as an analog phone. So it would connect up both channels, and when a call came in, drop down to one 64k channel. It was incredibly reliable. Even when I got DSL, I kept the ISDN for quite a while because it had more consistent upload speed.