BSNL - Bharat Sanchar Nigam Limited, is as per section 2(a) of RTI act a public authority and hence comes under the ambit of RTI. All Public sector Units, companies owned by either central government of state government come under RTI.
Only Public Sector Units like HAL, BEL come under RTI. Other Public Limited Companies are private companies and do not come under the ambit of RTI.
However, these companies have to provide some information to government bodies like SEBI and RoC( Registrar of Companies). You can ask SEBI and ROC this information. RoC provides lot of information about companies on their site, you need to pay some fees and can get information immediately.