Dear Mr.Deepak Wadhwa,
As a matter of policy the Telecom Companies do levy charges be it for premium numbers or VIP numbers. Otherwise also from prepaid to postpaid and vice versa they do charge migration charges. As a matter of fact you must have signed Subscriber Enrolment Form with the service provider kindly go through the terms and covenants of the same afresh as in case you have signed something on the dotted line then you cannot back out.
But otherwise as it is evident that initially you opted for the post paid connection with a VIP/premium number kindly see if in subsequent bills they have levied the charges for the same or not (as subsequently if in case you are disputing that might not be of any use as previously you have paid and not agitated).
Last but not least you have a strong case against the Company that if a subscriber who has a premium/vip number opts out from post paid to prepaid then whatever charges they are asking is liable to pay or not.
You escalate this matter through Regulator (TRAI) seeking explanation for the same in writing.
Best Regards
Daksh