They still exist - for businesses that never want to leave a call unanswered, but can't afford employees 24 hours a day.
Even at the time I believe it was mainly a tool of professional businesspeople - and actors, awaiting calls for auditions. And gamblers, of course. (See Bells Are Ringing!)
Also, doctors--and they still use them, as the service can sometimes make a judgment call about whether contacting the physician after hours is urgent or whether simply to refer the caller to whichever other doctor is "on call."
