Trunk versus toll telephony
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In the US, under the purview of the Bell System, local telephone calls were typically unmetered and included in flat rate billing. The term toll calling was adopted for long distance calls which were subject to toll charges.
In the UK, all calls were chargeable and the term trunk calling was adopted for long distance calls. Initially a trunk call had to be booked in advance and a switchboard operator called the subscriber when the call setup was completed. In 1921, the first toll exchange was opened which allowed the short-distance trunk calls to be connected while the subscriber waited. The name toll was probably taken from US usage.
In the London telephone area, calls to subscribers who were not serviced by Director telephone systems within local call-charging range of the London Director exchange area were carried by tandem exchange Toll A: a subscriber would dial a prefix code, usually three letters, followed by the number of the other subscriber on the fringe non-director exchange. Calls from fringe non-director exchanges to numbers within the director area were passed in two ways. For those to director exchanges near the area boundary adjacent to the non-director exchange, subscribers were given codes to dial, and told to follow the code with the numerical portion of the other subscriber's number. For director exchanges remote from the area boundary, the subscriber was told to dial 7, to wait for a second dialling tone, and to follow this with the whole of the other number. These calls were handled by tandem exchange Toll B, from which the second dialling tone originated. To have a second dialling tone in this way was very unusual on the PSTN in the UK, although the French PTT had a similar instruction for when their customers made international calls.
The distinction between trunk and toll became irrelevant when subscriber trunk dialling (STD) was introduced.