A telephone number (or subscriber number) is a sequence of numbers (less often also letters ) assigned to a user or a subscriber of the telephone network, knowing that you can call him. From a technical point of view, a telephone number is a necessary condition for automatic call switching, which determines the route of its passage and the search for the telephone equipment of the called user for the connection (as part of signaling). You can use various tools in order to identify the caller. The telephone number is assigned by the attendants of the PBX or switch, so that each user of the network has a unique identification. When connected to the public telephone network, the subscriber number (identifier, ID) is allocated by the service provider when entering into an agreement on the provision of telephone services. In turn, the relevant state and international organizations are involved in the regulation and distribution of ranges (blocks) of numbers in the global public telephone network between companies, as well as in standardization and common control over communication services.