Explain telephone numbering system of Russia ?
The telephone numbering plan of the USSR was a set of telephone area codes, numbers and dialling rules which operated in the Soviet Union until the 1990s. After the collapse of the USSR, many newly independent republics implemented their own numbering plans. However, many of the principles of the Soviet numbering plan still remain. Russia and Kazakhstan still retain the former Soviet international code +7.

Key principles
The Soviet Union used a four-level open numbering plan. The long-distance prefix was 8.

a).One could call a local number without the code. Local numbers usually consisted of 5-7 digits, with seven-digit numbers only occurring in Moscow, Leningrad and Kyiv. If the internal number of the regional centre had less than seven digits, then its intercity code was supplemented with numbers (usually 2 for the administrative center, 6 for the second largest city).
b).Within the same numbering area (most often within the state or region) the pattern was: 8 2X YYYYYY, where 2 replaced the three-digit area code.
c).For calls to other areas, one had to first dial long-distance prefix 8, then, after the tone, the full code of the numbering area, which consisted of a three-digit code and zone additional digit(X), and then the local phone number.
d).For international calls, one should dial 8 10 [country code]  [phone number]
For example: 8 10 1 212 XXXXXXX for a call to New York City.
Also: 8 10 359 2 XXXXXX for a call to the city of Sofia.