The vast majority of modern services in the United States use toll-free numbers whereby clients can dial up to request a call with a particular performer using credit cards, Automated Clearing House systems, and a variety of other billing methods.
There are still some services that rely upon premium-rate telephone numbers (e.g., 976 and 900 numbers) for billing purposes, although this practice has been largely abandoned due to the high rate of fraud associated with these lines and the inability to dial 900 and 976 lines from cellular phones.
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.
With the progress of technology it became more practical, convenient, and economical for providers to work out of their homes.
Human dispatchers — female, except for gay male phone sex — answered the advertised phone numbers, processed payment via credit card, chose who of the available performers in the dispatcher's judgment best matched the clients' fantasy (grandma, black girl, college girl, etc.), and connected the client with the provider. Either could hang up, though some services put economic pressure on providers not to do so.
The provider provided (say) 10 minutes of service, but got to keep all of the money (say 20 minutes).