Originally, per-minute billing was provided by phone companies (in the U. There was, from some services, an attempt to keep the caller aroused but short of orgasm, so he would spend more money.
(This attitude still survives among some providers.) When public (mostly female) pressure forced the phone companies to stop providing this service to sex workers, a transition was made to a manual method: pre-paid blocks of time, 10, 30, 60 minutes, whatever the customer would pay for.
Verizon provided billing services to calls made in Massachusetts, Rhode Island and Maine.
AT&T and MCI offered nationwide collection services, with a cap of $50 per call.
The provider provided (say) 10 minutes of service, but got to keep all of the money (say 20 minutes).