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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.
Originally phone sex services consisted of a managed network of dispatchers (live or automated) and erotic performers.
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.
Details vary significantly from one platform to another, but the provider may be given a personal page on the platform to use however she (sometimes he) wishes.
All have some way for a provider to post a picture and some text.