Throughout the history of telecommunications, most efforts have been aimed at encouraging people to use the service more. But in wartime, different rules apply.
The demands of military security and national priorities meant they had to be urged to use the service less - and with more care.
In the early years of the Second World War, the extent of spying activity taking place in the UK was unclear. The country was urged to be cautious about giving secrets away into enemy hands.
People were warned by a series of posters that thoughtless remarks could cause devastating results on the battlefront. 'Careless Talk Costs Lives', ' Loose Lips Sink Ships' and 'Walls Have Ears' were famous catchphrases of the day.
War made heavy demands of the GPO's services and it was keen to make sure everything ran smoothly for the essential work of the war. As well as suspending the Greetings Telegram service, people were urged to make less demand on the system with a series of posters that coined new phrases, such as: 'Don't Make Trunk Calls', 'Telegraph Less', 'Telephone Less' and 'Send it by letter not telegram - I'm busy I'm on war work'.
Marjorie Chapman, was born in Willesden Green, north London in 1920 to an everyday working class family. Neither she, her family or friends had a telephone and she didn't actually use one until she joined the GPO as a telephonist in 1935, when she was 15 years old.
She enjoyed the job but a few years later, during the war, her commitment was truly tested working at the Faraday Building.
The accommodation provided during air raids was rudimentary but better than hazarding the bombs on the journey home. The work was lightened by the actor Norman Wisdom, who worked there then and told stories during the shifts.
Play the audio clip and view the transcript to learn more. A new window will open.Jean Elliot was born in Harrow, north London in 1924. She had had no experience of the telephone until at the age of 17 she joined the GPO, where her cousin already worked.
Jean worked at the telephone exchange throughout the Second World War, manning the switchboards.
She remembers working during the air raids of the Blitz, in 1941, without being evacuated or taking shelter from the bombs because, as she says, they were just too busy.
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In the 1930s a new generation of '300 series' telephones was introduced. These came in the usual black, green, red and ivory but were developed to provide a range of other capabilities too.
A black telephone with a green handset meant only one thing - 'Secret'. These were the 'hot lines', widely used by government ministries, the military and secret services. They were connected to a large box kept on the floor (replaced by a much smaller version in the 1960s), which was used to scramble conversations so that anyone listening in wouldn't be able to understand a word, unless they had a similar scrambling box at their end as well.
A caller pushed the button on the left-hand side marked 'secret' to activate the device; otherwise by pushing the 'normal' button it worked like any other telephone.
This phone was provided for the private use of Queen Elizabeth in her bedroom on visits to Dower House at Arundel Castle. The unit combined a black based telephone with green handset with a Privacy Set No.6 as a scrambler system. It was taken down to Arundel and installed each time there was a royal visit and allowed secret conversations to take place between Arundel Castle and Buckingham Palace.
This system worked on a normal telephone line but the box contained a number of transformers and valves that scrambled the speech, so if anybody tapped onto the line (without a de-scrambler) they would hear gibberish. The user at the other end of the line had to have similar equipment, to unscramble the call.
