Re: I saw similar a couple times in that timeframe ...
My recollection, because I started to make phone bill payments in those years, was that the local operating telcos (first the “Baby Bells” and then their ever-merging successors) had two types of residential service on offer: one at a nominally lower base cost plus a charge for every local call, and one at a supposedly higher base cost that allowed unlimited local calling. Both, of course, charged a king’s ransom for a domestic long-distance call. An overseas long-distance call required a cardiologist when your bill arrived.