Ex-AOL employee checking in here. Some phone companies charge per local calls, some don’t. It generally depends on the area you live in.
At the same time AOL pays the backbone provider X amount of dollars per gigabyte sent over the lines. The backbone provider then pays the local phone company based on the amount of data the line tranmits. When you are idle on AOL or any other ISP the ISP sends a heartbeat to your computer. The heartbeat basically asks the users computer “Are you still there?”. If the users computer replies “Yup” the connection stays open. Since the heartbeat packets are really small keeping the connection open doesn’t cost all that much.
So AOL is paying a tivial amount to keep your connection open. If you look around some ISP’s are advertising a flat rate that also has a limit to the amount of data that can be transmitted over the connection per month. IIRC, AT&T implemented a limit on how much data could be transmitted using their unlimited plan. People were not happy with it but they had to follow the plan.
In other words, it is not how long you are connected that matters it is how much data you send and recieve.
Slee