Airtel — my unbilled amount reached rs3000 where as my credit limit is just rs500

Suddenly after using Airtel for almost 3 years I got a shocking Unbilled amount of Rs3000 without any extra usage without ISD or IR usage. Even Data is also available and no VAS activated. Had my Credit Limit of 500Rs and still today I see my Unbilled Amount as Rs3000. Customer care has no response other than "pls wait for bill to generate".

Why? Why would I wait for bill generation? If Airtel can't explain Unbilled Amount before bill generation then why they are showing to customers?

If Credit Limit can't stop usage going above it then why Credit Limit is made in Airtel?

If there is someone from Airtel who can resolve this issue then please reach me. It's not at all comfortable to see such huge amount o[censored]nbilled if you have not used anything extra other than plan benefits.