Airtel Prepaid — Not allowed to make calls to sister's mobile

Address:

Bangalore, Karnataka

Dear Sir/madam,
two days back, i got a new airtel sim for my sister with number [protected], and it was promptly activated within 2 hours. But when i call her number from my airtel mobile, the recorded message says 'you are not allowed to call this number'. i tried calling from my father's and mother's airtel numbers, but get the same recorded message.
The strange this is, when we call from my brother in law's vodafone mobile and our bsnl landline, the call is going through. she is also able to make outgoing calls without any problem.
Please check this asap, as it is leading to severe communication problems for us.
Thanks
Sandeep
Ph: [protected]