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 incentives for providers were then reversed; rather than earning money from keeping the customer on the line (orgasm delayed), they earned more from bringing the caller to orgasm quickly, so as to move on rapidly to another call.
Unused minutes were rarely usable on a second call.
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.