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The telephone companies would provide billing services for chat line companies.
Typically the telephone companies would bill callers to chat lines and then remit 45% of the money collected to chat line operators.
When the Internet got relatively mature, sale of any sexual service not involving a minor could be made to anyone not a minor.
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.
Performers would come to a studio where they received a cubicle, coaching, and cash incentives to keep callers on the line longer.
This is the world portrayed in Spike Lee's movie about phone sex, Girl 6.
Big platforms as of 2016 are Niteflirt, Talkto Me, and My Phone Site; the latter also includes provision by which a manager, with the consent of the providers, could have a virtual shop with many providers under them. Customers had a variety of payment options, and pages of providers to choose from, sometimes with voice samples available.
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.