It was thought the normal public switched telephone network would be severely affected by affected by enemy action. In order to preserve what was left intact for use by emergency services engaged in the recovery operation a scheme was put in place to deny nonessential customers access.
Background to the UK Analogue Telephone Network
Following the Nationalisation of the telephone companies in 1912 the UK telecoms infrastructure was run by the GPO / Post Office Telephones and in 1984 an Act of Parliament changed it into a public limited company British Telecom (BT). Prior to that the GPO analogue network was very different from today's digital networks. There was no competition from operators in the market such as Mercury, Sky, TalkTalk or Virgin, as there is today. The GPO was the sole provider of telephone communications for the public and military.
At the start of the cold war, less than a quarter of people had a telephone in their own home, there was often a waiting list for a telephone, two residential customers may have had to share a line, with no privacy between them. Unbelievably, there were advertisements to discourage calls at certain times of the day and to encourage users to keep the calls to a short duration.
Many towns still had a manual system, requiring the user to pick up their handset and ask the switchboard operator for the number they required. Over 2,500 of these UAX 13 exchanges were in service by the mid-sixties, typically found in villages providing lines for up to 400 customers, it offered automatic dialling facilities.
In areas with automatic exchanges, local calls (classified as within a radius of 20 miles) could be dialled directly. Automatic long distance dialling known as Subscriber Trunk Dialling (STD) was introduced at Bristol in 1958. Full national rollout took in the region of 20 years. Before STD, automatic exchange customers wanting a call over 20 miles dialled either '0' or '100' for the operator who then connected the call. Even after the introduction of STD it was a number of years before every part of the country could be reached, requiring calls to those other places still to be connected by the operator.
Major towns had an analogue automatic telephone exchange known as a Group Switching Centre (GSC). This exchange switched customer dialled calls within the local area. It housed the group routing and charging equipment for STD calls within that same area. Often there would be a co-located Auto-Manual Centre (AMC) housing operator switchboard positions handling long distance calls that couldn't be dialled directly, directory enquiry and 999 calls for the local area.
During the nineteen seventies, home telephones gradually became more common, so by the eighties the majority of homes had one. Analogue mobile telephones the size of a brick only started to appear in the early nineties just as the Cold War was ending, but the high price of handsets and calls restricted their use to mainly business people.
Starting in the mid eighties, the BT network was slowly modernised from an analogue to a digital system, culminating with a totally digital network by 'Phoneday' on April 16th, 1995. The cold war having finished a couple of years earlier.
Predicted Wartime Telephone Network Overload
All telephone networks are very susceptible to overloading caused by a local or national event, this can be anything from bad weather to a radio phone-in. The networks are only designed to cope with average number of calls. With today's modern digital networks, calls can be rerouted to avoid local problems. During the Cold War period the analogue network didn't have this resilience.
It was assumed overloading would occur during the period of build up to war as people tried to call relatives. Damage caused by war action itself would severely disrupt the network. To prevent overloading affecting essential services and the war effort, a system of disconnecting less essential telephones was introduced in the analogue network, which has been carried forward into the digital and mobile networks too. The disconnection facility is known as the Government Telephone Preference Scheme (GTPS).
This may sound harsh but in the sixties and seventies the network often could not cope with normal traffic and engaged tone was very common. In a war situation it was unlikely to cope with the predicted surge of calls, so by invoking the GTPS, the capacity could be reserved for government, military and civil defence lines
At the time of the Electricity Generator workers' industrial dispute in the early 1970's I recall working by battery lights during a power cut to match the correct level of preference category for emergency lines, compared with the list provided by the 'Area War Group' (AWG). However it was never necessary to use the Preference Scheme in Kettering as our emergency generator never failed.
Cold War - Government Telephone Preference Scheme
This scheme works by separating telephone lines into three preference category groups (Pref Cat).
Preference Category 1 (Pref Cat I) lines to essential civilian services (Police and Fire stations), Local Council / Regional Government bunkers, UKWMO Bunkers, military establishments and the private phones of critical post holders.
Preference Category 2 (Pref Cat II) lines to Public call boxes, public utilities (Gas, Water, Electricity), Schools as Feeding Centres and council premises.
Preference Category 3 (Pref Cat III) comprising of the majority of residential and business lines.
The first stage of disconnection prevents Pref Cat III customers from making calls. The second stage of disconnection prevents both Pref Cat II & III lines from making calls. The Pref Cat I lines have no disconnection facility. All lines regardless of their category can receive incoming calls. So even with the maximum level of disconnection enabled, a Pref Cat I line would function normally as they could still make a call to a lower priority line with Pref Cat II or III.
The photo shows the preference control slide in units at a TXE2 exchange on display at Avoncroft Museum, Bromsgrove. This type of electronic exchange with a reed relay switching matrix was introduced in 1968. The preference keys are mounted on the top shelf of the Calling Number Generator rack, disabling its function in allocating customers to the equipment that receives the number dialled, thereby stopping them making calls.
The red keys have little spring plates to stop them being accidentally operated. The nine keys in a group are the local control of the preference level. The other unit with a single 'Test' red key is the remote control unit, allowing the preference level to be set firstly by calling an ex-directory number to prime the system and then dialling a second ex-directory numbers within three minutes to switch the preference on or off. In all cases, the master key, top left on the group of nine must be operated to enable both local or remote control.
Within each GPO Telephone area, a small group of employees in the 'Area War Group' (AWG) liaised with government and the military to maintain a list of lines in Pref Cat I & Pref Cat II. When these lists changed, engineers were instructed to take action to modify the customers exchange line in the exchange.
At the time of the Electricity Generator workers' industrial dispute in the early 1970's I recall working by battery lights during a power cut to match the correct level of preference category for emergency lines, compared with the list provided by the AWG. However it was never necessary to use the Preference Scheme in Kettering as our emergency generator never failed.
Telephone Service Restrictions, Post Cold War
Even as network capacity has increased to present day standards, voting for a reality television shows can easily overwhelm it. In the present day digital network an additional software tool known as call gapping is used to reduce the peaks of calls to a specific number(s). This wouldn't work in a war emergency situation where calls are made to friends and family but not to one specific number. The GTPS still exists in the landline network and a similar arrangement, the Mobile Telephone Privileged Access Scheme (MTPAS) is in place on the mobile phone network. They may be used if we suffer a terrorist attack or a natural disaster such as flooding or earthquake jeopardises the telephone network.
Even after the end of the cold war, the GTPS and MTPAS for mobile phones still exist and are likely to continue in one form or another. This should not been seen as anything sinister. In the past it has been necessary to use the GTPS when telecommunication equipment has been accidentally damaged to ensure the emergency services can still function.