Are you using Airtel on your Blackberry? If yes, you will understand that all Airtel Blackberry plans have data caps. Even the so called unlimited plan has data cap.
A user that downloads very well can consume Airtel weekly and or monthly data plan before its validity period. If you subscribe after consuming the allotted data size before its validity period, you will receive a message back in return that ‘Dear subscriber, your request was not successful because you cannot migrate while you still have an active service.’
For your request to be successful, you have to deactivate the existing Blackberry plan and then re-subscribe again. To deactivate it, send DEACTIVATE to 440 via sms.