Subscriber Trunk Dialling

On 5th December 1958, Her Majesty The Queen launched the STD (Subscriber Trunk Dialling) system in the UK by making a call from Bristol Central telephone exchange to Edinburgh Caledonian, dialling 0 31 CAL (225) 3636. This connected her directly to line 3636 on Caledonian exchange (located in Rose Street), a distance of some 300 miles away as the crow flies. Up to this point, all long-distance calls (so-called "trunk" calls) had to be connected by an operator, even if the caller and recipient were both on automatic exchanges.

At the ceremony, in the Strowger apparatus room at Bristol Central exchange in Telephone Avenue, the call — from Her Majesty to the Lord Provost of Edinburgh, The Rt Hon Ian Johnson-Gilbert — was relayed to those present over loudspeakers. Immediately after making the call at 3.33pm, the Queen flicked a switch which connected all subscribers on Bristol Central to the new STD network. She was then presented with the telephone she had used, a colonial blue coloured prototype of the new 700 series, due to be introduced to the public the following year.

After the ceremony, the Queen & The Duke of Edinburgh were given a tour of the new equipment at Bristol, which provided the new service, by Ernest Marples MP, Postmaster General, along with the Lord Lieutenant of the County of Bristol, the Mayor of Bristol, and other senior Post Office staff.

Bristol was chosen in 1955 to be the launch city for the new service, as it generated and handled a fairly large amount of trunk (long-distance) traffic, with direct routes to many other cities. Bristol Central was already a "Zone Centre" on the existing trunk network, handling long-distance calls to and from the South West zone of the country, and the exchange was already planned to have adjustments to the existing equipment which made interfacing with STD easier. The choice was therefore made to have STD ready at Bristol Central at the start of 1959, and to extend it to other exchanges in the Bristol area during 1960.

What is STD?

Automatic exchanges started to be introduced from 1912 (see our Phone Switching page), allowing callers to dial numbers without the help of the operator — but only for local calls, those to people on the same exchange or a nearby exchange. Long-distance calls, known as trunk calls, were still switched manually.

With around 6,000 exchanges in the country, it's impractical to directly connect them all together. Long-distance calls therefore make use of intermediate (trunk) exchanges. Up until digitalisation in 1990, the trunk network in the UK consisted of a number of Zone Switching Centres (ZSC) — big manual exchanges which handled all traffic in and out of that zone. All 18 zone centre exchanges were fully interconnected — each one had direct links to all the others. Each zone was subdivided into smaller areas consisting of a group of local exchanges (those with customers directly connected to them). Each group had a main exchange called a Group Switching Centre (GSC), typically located in the principal town. A call within a group would be handled by the GSC; if it was for a different group in the same zone, it might go direct between the two GSCs, or alternatively via the ZSC (GSCs would be directly connected if traffic levels warranted it). Calls outside the zone would go from the GSC to the ZSC.

Prior to STD, although many local exchanges were automatic, the GSCs & ZSCs were still manual exchanges. After the introduction of Subscriber Trunk Dialling, the caller could dial right through the network to the distant local exchange.

STD thus required new equipment at the GSCs and ZSCs, together with a new national numbering plan to identify call destinations. The numbering system introduced is still the basis of today's numbering system, with the majority of area codes unchanged, apart from the introduction of a 1 after the 0, which was done in the mid-'90s. Some areas, such as London and other cities, have had new codes to provide additional numbers to cope with growth in subscribers, but 31 (0131) still identifies Edinburgh.

Numbering scheme

The first change was to move access to the operator from 0 to 100, allowing 0 to be used as the initial digit to indicate an STD call.