India's national numbering format is 10-digit for both land line phone as well as cellular phone services. This 10-digit format is expected to remain for another 30 years.

The subscriber's phone number is 6, 7 or 8-digit long depending upon the length of the STD code. STD codes are either 4, 3 or 2 digits.

When the area code is 2-digit, the phone number is 8-digit (2+8)
When the area code is 3-digit, the phone number is 7-digit (3+7)
When the area code is 4-digit, the phone number is 6-digit (4+6)

For example, all metropolitan cities have 2-digit area/STD code and 8-digit phone number. At present, 2-digit STD codes (also known as area or trunk codes) are 11, 20, 22, 33, 40, 44, 79, 80.

First digit of the Phone Number identifies the telecom operator :

BSNL/MTNL (Government) - 2
Reliance - 30, 31
Bharti - 50, 51
First digit in subscriber number will not be 0, 1, 7, 8 and 9.

Cellular phone number

For cellular phone services, the numbering format is
2-digit access code (currently 98 or 94) plus
3-digit provider code plus
5-digit subscriber number.