In the USSR, local telephone calls were unlimited, while long-distance calls required a centralized billing system. To charge for long-distance services, the network needed to identify the caller's number, the destination, the duration of the call, and the subscriber's category.
Before the widespread adoption of automatic number identification (ANI) equipment, the network used a temporary method called 'own number dialing' (NSN). Subscribers had to manually dial their own phone number after the long-distance signal, allowing the exchange to verify the caller's identity via a 1600 Hz tone.
Electromechanical exchanges could not store the caller's identity during connection setup. Dedicated ANI equipment was installed to physically identify the subscriber line by connecting wires from each subscriber set to a matrix. This hardware was designed to handle specific traffic loads based on the number of subscribers in a group.
The ANI system operated by sending a 500 Hz request signal from the long-distance exchange. Upon receiving this, the local exchange identified the line and transmitted the number using a '2 out of 6' multi-frequency code. The number was transmitted in reverse order, starting from the units digit, in a continuous cycle of 13 combinations.