In concept they have a lot in common with platforms such as Ebay: the seller provides the picture(s), description, and sets the price, a percentage of which is kept by the platform.
In the sex industry, similar platforms emerged facilitating the selling of used panties and other odoriferous garments, and for "cam" video sessions, in which the customer, for a fee, can direct the woman on the video screen, and for a higher fee, have a private connection (no one can see caller or provider except each other).
At that time independent phone sex was more dangerous, as Lee's movie portrays.
With the progress of technology it became more practical, convenient, and economical for providers to work out of their homes.
Human dispatchers — female, except for gay male phone sex — answered the advertised phone numbers, processed payment via credit card, chose who of the available performers in the dispatcher's judgment best matched the clients' fantasy (grandma, black girl, college girl, etc.), and connected the client with the provider. Either could hang up, though some services put economic pressure on providers not to do so.
The incentives for providers were then reversed; rather than earning money from keeping the customer on the line (orgasm delayed), they earned more from bringing the caller to orgasm quickly, so as to move on rapidly to another call.
Unused minutes were rarely usable on a second call.