> Until some makes a call or sends a fax when your top speed drops 64k per phone line being used.

That wasn’t how it worked. You could have a T1 with certain channels configured for voice and others for data, but the configuration was static — you didn’t get to use the bandwidth from the voice channels when they were idle.

> It’s also why ISDN didn’t connect at 128k but at 112k since it just used two “bonded” lines of the T1

ISDN bearer channels were 8-bit clean. If you bonded both channels of a BRI, you had a 128kbps data path. What you typically couldn’t use was the 16kbps D channel, which was used for signaling.