I remember asking T-Mobile about unlocking my phone except you have to spent something like $100 I think their network through phone usage before they will even unlock it which is different from before when you only had to be using it on their network for 3 months before it was done for free!
Sorry, I think you will have to find a business that can possibly unlock it for you because T-Mobile won't because of their policy noted above for it to be unlocked for free...Also, the phone is only worth $5 for trade-in value otherwise I would suggest that it maybe cheaper going on Ebay and purchase one that is already unlocked because you may find it will be cheaper than paying to get your current phone unlocked which is ironic knowing that a brand new Nokia 521 from a shop retails now at $49...locked to T-Mobile!