Until the 1970s the Bell System had only two toll rate types --
-station and person. All station calls, whether dialed direct or operator handled were charged the same price. In the 1970s, with virtually the whole country equipped for DDD, the Bell System began to give discounts for directly dialed station calls, particularly at night hours. Over time the directly dialed calls got cheaper while operator handled station calls got more expensive. This was a big change at the time, rolled out gradually. I think it was test marketed in North Carolina. I believe at the time it also introduced a late night toll class (after midnight, then after 11pm) that was cheaper than the existing evening rate. We college students, who kept odd hours and had friends far away, made good use of it, but you had to call from a home phone, not a coin phone. Coin rates were "operator handled".
Station calls included plain, "time and charges", 3rd party billing, collect, and 3 minute notification. In the 1950s people could call by name and city "get me John Jones in Seattle" and the toll operator would first get the number from Seattle D.A. The Bell System encouraged people to call by number to save operator time.
Anyway, there was no difference between sent-paid and collect rates. Enterprise calls saved the trouble of an operator calling ahead to get acceptance, that was assumed.
My point was that in the 1980s the cost difference between an 800 service and Enterprise service began to be significant. By 1985 it was foolish for a business to maintain an Enterprise line. Perhaps they didn't even know they had the service and the collect calls were just buried in with other toll charges.
[TELECOM Digest Editor's Note: Do you recall how, even after they set up a reduced rate for 'direct dialed' calls versus a more expensive rate for 'operator assisted' (but still station to station calls) if a customer appealed the higher 'operator assisted' rates the customer would be given 'direct dial' rates if he had 'tried to dial it himself but failed' for some reason? PAT]