One of my friends gave me this link to register complaint about BSNL service.
http://mis.bsnl.co.in/pgs/internet/pgwebregn.asp
I had a complaint with my
broadband 
connection that it became slow and started to drop frequently. I registered a complaint in this page and within 1 hr got a call from the exchange. Now, after 2 hours, they came to my home and fixed the problem.
Hope everyone using BSNL could avail this facility.