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AT&T and MCI offered nationwide collection services, with a cap of per call.
The vast majority of modern services in the United States use toll-free numbers whereby clients can dial up to request a call with a particular performer using credit cards, Automated Clearing House systems, and a variety of other billing methods.
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.
Unused minutes were rarely usable on a second call.
The provider provided (say) 10 minutes of service, but got to keep all of the money (say 20 minutes).
When the Internet got relatively mature, sale of any sexual service not involving a minor could be made to anyone not a minor.
Software platforms were custom written to handle money collection and transfer, connecting caller and sex worker though neither could see anything but the platform's phone number, and metering the connection.