Airtel — unable to call number starting with 1800 from my prepaid number

While trying to call to any toll free number starting with 1800 i'm experiencing issue. It told me like invalid number kind of. Where as the same toll free number i'm able to call from my friend's mobile. Several times i called customer care.. But it is not resolved. One month back some customer care executive received my call but told me to look into the matter. But i have not received any solution. I'm still facing the same issue. I have been using this prepaid number since 2010 from airtel. But if they do not resolve my i have to port my number to other vendor to get a solution.