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).
All have some way for a provider to post a picture and some text.
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.
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.
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.
The sexually explicit conversation takes place between two or more persons via telephone, especially when at least one of the participants masturbates or engages in sexual fantasy.
Phone sex conversation may take many forms, including: guided fantasy, sexual sounds, narrated and enacted suggestions; sexual anecdotes and confessions; candid expression of sexual fantasies, feelings, or love, and/or discussion of very personal and sensitive sexual topics.