The IMEI is blocked for either non-payment to T-Mobile or was reported stolen.
You will need to work with whoever sold you the phone to try and correct the account problem that is causing the issue.
That is the only way to get them unblocked. If the original purchaser won't fix the problem or you can't get in contact with them there is nothing that you can do besides that to fix the problem.
You can check IMEI status with t-mobile at http://www.t-mobile.com/verifyIMEI.aspx