Not only the BSNL is below on the total number of wireless connections to Airtel, Vodafone, Reliance and Idea, it is also much below all of them in active connections also.
The total wireless connections as in March 2013, the total connections and active connections are given below: % of active connections given in brackets.

Airtel 188,196071 ( 24.73%), Vodafone 152,353,654 (20.15%), Reliance 122,972,717 (14.76%), Idea 121,607,390 (16.63%) BSNL 101,206,625 (7.53%) MTNL 4,999,351 (0.32%).

The less number of active connections may be due to the government and other official connections. MTNL, which functions in Delhi and Mumbai alone has got only 0.32% connections.

However, this is a matter to be examined and the reason found out.