A trick some ISPs used in the 90's was a "data over voice" call, which ran at 56K but was charged voice rates instead of data rates. That meant the call was generally free. The improved latency of ISDN made a huge difference compared to a 56K modem.

True, the nominal 8 k weren’t much of a change but the cut down on latency made a big difference especially while „surfing the web“.