Originally phone sex services consisted of a managed network of dispatchers (live or automated) and erotic performers.
Due to the potential for emotional intimacy between those who have engaged in phone sex, it is a matter of some debate whether phone sex is to be considered infidelity when involving a person outside of a committed personal relationship.
Nevertheless, phone sex should not be confused with prostitution wherein money is exchanged for real life sexual services or physical interaction.
The editor of High Society magazine, Gloria Leonard, is credited with being one of the first people to use "976 numbers", then "900 numbers" for promotional purposes and soon as a revenue stream in the adult industry.
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).
This is the world portrayed in Spike Lee's movie about phone sex, Girl 6.