The first phones were permanently installed. In other words, they were attached to a particular house. True, it was very expensive, so the first such devices appeared in wealthy people and in medical and administrative institutions. Since at first all phones were directly connected, a single contact center was created that could later serve many phones at the same time and connect them to organize a call.
Initial numbers were randomly generated. This was possible because in principle there were very few. But as soon as the need and possibility of making calls to other countries arose, a single standard for the formation of numbers began to be established. Each number was eventually given a code indicating the country. Thanks to this, all numbers have become unique, that is, there is no analogue of this or that number in the world.
The International Telephone and Telegraph Advisory Committee worked on the formulation of recommendations for the establishment of telephone numbers. When creating a number, several rules are taken into account. For example, any number must begin with a + sign, a unique country code must consist of 1-6 digits, and the entire number must be 11 digits. Also, thanks to the code +8, you can call China, and the Russian code – +7. Only 7 digits of the number are unique for each subscriber. This is explained by the fact that all landline numbers once consisted of a similar number of digits.
Source: Ferra
