Someone probably sold you a phone prior to not making full payment or sold you a phone prior to it being stolen
doutorgabriel, I am very sorry you ran into this situation. tidbits is likely correct here; since our database isn't updated until an issue is reported, it's possible that the phone was stolen but not reported immediately, or that the original owner sold the device and then defaulted on the remaining balance. I know you've had a few other threads here, but I'm sorry that I didn't glean from those that the device you purchased was IMEI blocked in addition to being locked to the T-Mobile network. Contacting the retailer you purchased it from is going to be your best bet to see if there's any resolution they can offer, since we can unblock the device for the original owner if the phone is reported found or if the payments are settled, and after that we'd be able to unlock it according to the eligibility requirements here: Unlock your mobile wireless device.
This is a huge bummer and I'm so sorry that someone sold you a phone that you're not currently able to use. I hope that they'll do the right thing if you contact them and replace the device. I know that the pricing offered by individual retailers is often much more affordable, but your best bet for purchasing unlocked equipment would be to get a factory unlocked device from the manufacturer themselves -- in this case, directly from Apple. I'm crossing my fingers that the shop owner you got the device from will be understanding and that an exchange is an option.