Context

Till now, mobile numbers were traditionally registered to series starting from “9”, and some combinations starting with “8”, “7” and “6”. Currently, with these combinations a total of 115 crore mobile numbers are available. Even though the “9” series — meaning mobile numbers starting with “9” has been fully used, those starting with other numbers overlap with certain numbering plans issued to landline phones.

In news

Callers in the country using landlines to make calls to mobile phones will be required to add '0' as a prefix from January 15 onward.

Details

• The practice of dialling 0 before entering a mobile number was initially used for outstation mobile numbers until the central government permitted inter-circle mobile number portability thereby effectively making all mobile numbers local. 
• The latest decision will have no impact on mobile to mobile dialling procedures.
• India uses 10-digit mobile numbering plans, with numbers beginning with '1' and '0' unavailable to the public, this means that there are, theoretically, 800 crore possible numeric variations.
• Currently, mobile phone numbers are registered to series beginning with '9,' '8', '7' and the rarer '6.' 

Source: Indian Express