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).
Once means of transmitting payment were developed, phone sex turned into primarily a commercial activity, with customers (overwhelmingly male) and sellers (overwhelmingly female).
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.
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.
Phone sex is a conversation between two or more people on the phone where one or more of the individuals is describing the act of sex.
Phone sex takes imagination on both parties' part; virtual sex is difficult if the operator does not put the images in the head of the caller and the caller must be open to the pleasure as well.
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.