In the mid 1960s Western Union introduced a private line voice service called "Hot Line". In essence, a person lifting the receiver of one telephone would cause a specified distant telephone to ring over a private line. The connection was faster and cheaper than placing a conventional long distance call over the Bell System. WU charged by 6 second increments and at a lower rate; the Bell System at that time had a 3 minute minimum. WU says their arrangement was cheaper when more than 3 calls a day were made.
The connection between the two telephones was actually not a dedicated private line, but shared use of the WU network via concentrators. If a circuit was busy there were alternates.
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Obviously this service had some limitations since it was telephone-set to telephone-set. I don't think this could terminate in a PBX system to allow shared use of the line by a whole organization which would give more flexibility. I don't know if WU permitted any kind of multiple extension sets at the subscriber since a specialized telephone set they provided was used. For example, a secretary might want to answer the boss's hot/line phone if he was out.
WU also reported customers wanted to get the service in more cities than available.
None the less, it seemed like a pretty good idea for its time.
Would anyone know how successful this service was and how long it lasted?