Reg voice or 2g coverage---
AFAIK BSNL's only profitable circle is Kerala. So you can use BSNL there as plans are cheaper as well. Many years back [minimum 14] when we went to Munnar, and the only telecom operator there was BSNL. Later others may have surely catched up, especially idea who bought the old operator escotel there and Vodafone who got
bpl [loop] there. Kerala is weak point of Airtel, since it is the only circle where they don't have own 3g license till now. In Kerala, Vodafone, idea, BSNL are using 900MHz spectrum for 2g and voice calls. Airtel,
tata, rcom, aircel are using 1800MHz for 2g and voice calls. Since it's mostly a hilly region, 900MHz guys rules there.
Reg 3g, tata, idea, aircel and BSNL has own license. Vodafone got own 3g license in this year auction. Reg icr, Airtel, Vodafone are using/sharing idea's 3g network for time being. Rcom uses tata's network, as it's favorite partner aircel is in coma stage there. Tata has 3g coverage in kottayam, but spotty at best. So except aircel, everyone is OK in the order of idea, BSNL, Vodafone, Airtel, tata, rcom.
Reg 4g.. Not sure. Airtel has own 20MHz bwa [2300MHz band 40] spectrum there, bought from
Qualcomm. Reg 1800MHz liberalized spectrum on band 3, idea has 10MHz, Vodafone has >9 MHz and Airtel with 5 MHz, but idea and Vodafone are yet to launch there and I am not sure about Airtel wrt 4g. You may need to ask cc.