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In the Moscow Telephone Network Subscribers Directory published in 1916 the first article of the rules of use reads as follows: The central station is called by taking the microtelephone off the hook.
The operator at the station should answer by telling her number.
Mnemonic rules were in use in London and Paris until mid-1960s.
At first Americans adopted the LLL-NNNN format (three letters, four numerals).
In 1920 Sovnarkom (the Council of Peoples Commissars) decreed that all telephone sets be seized from private individuals owing to a dire shortage of numbers.
Automatic dialing was possible with a rotary dial telephone set.
The last thing for the USSR government to get worried about in the 1920s was creating a system of mnemonic rules to make the memorizing of telephone numbers easier.
At that time numbers like L-NN-NN (one letter, four numerals) were pretty common.
Later an automatic telephone switch came into use that had two-letter indices.
In the 1910s automatic telephone switches started to supersede hello girls who made connections manually.
By the beginning of the First World War the USA had over 100 automatic telephone switches, Germany7, Great Britain2.
For historical reasons zero was always used to call the operator.