This is the world portrayed in Spike Lee's movie about phone sex, Girl 6.
At that time independent phone sex was more dangerous, as Lee's movie portrays.
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.
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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 types - young, mature, grannies, BBWs & housewives.
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.
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.