Sorry, I missed "blackberry" in my previous reply. I do have a blackberry dataplan(6 month unlimited plan).
I tried to downgrade it to earlier version, but that didn't fix the issue. Then I asked the customer care guy to reinstall the OS for me. He told that the software they have wont work in my case as it is from T-Mobile.
I will try to downgrade my OS.
As someone pointed out earlier, there is no difference between different OS provided by different service providers, then why does all the service providers provide the link for the OS to download. Everyone can dload the OS from blackberry site itself. Just curious.