Submit a fitting reply to the legal notice preferably by redg. post and maintain proper record.
In case there was deficiency in service, overcharging or intimidation etc lodge a complaint under proper acknowledgment in local police station.
TRAI has issued guidelines on consumer protection, FAQ, quality of service standards etc.
As per notification issued by DOT the confusion created by lawyers of telecom companies that Consumer Forum can not decide consumer cases has been removed.
You can download it from:
https://www.lawyersclubindia.com/forum/Consumer-court-can-now-decide-telecom-complaints-100830.asp#.U0wKFEeBmXU
List of NGO's is published at TRAI website.
There are consumer help groups that can help.
https://www.mouthshut.com/
complaint@network18online.com
pehredar@moneycontrol.com
pehredar@network18online.com
https://www.facebook.com/Pehredar
www.bighelpers.org
https://ashisha.com/consumer/
www.grahakseva.com/
https://www.akosha.com/contact.html