I did try putting another working sim in my blackberry device, but it didn't work in my device.
When I went to the customer care center after calling them for a week, the customer care guy tried my SIM in a spare blackberry they have and it worked in the spare blackberry. After that he tried my device with a spare SIM they have for testing, but it didn't work for my device.
He told me that there might be some problem with my device and as the blackberry handset is from T-Mobile he denied to fix it.
I have no idea what might be the problem.
Thanks for the replies,