It may have changed now, but back in the day it was assumed to be more expensive to terminate a phone call than to originate it, and telco’s would track minutes originated vs terminated and settle up each month by paying an “imbalance” fee. This was mandated by the CRTC in Canada.
The CRTC forced the incumbent telcos to let small players resell local phone service, and there was legit competition for long distance. What does this have to do with dialup?
Smart ISPs would setup as full telcos, with a small phone subscriber base. But the real money was all the people who got a second phone line from the incumbent telco and left their computer dialed up 24/7 (this was me!).
The “imbalance” revenue from all those dialup calls you terminated could exceed direct subscriber revenue from your dialup customers!