Two issues. First, each postpaid T-Mobile line is limited to two unlock requests per year, so that's quite a favor you're asking. Second, T-Mobile will want to see usage of that phone with the SIM in question before approving the unlock request. So, the phone and the SIM would have to physically get together. Maybe you should discuss this with the person you got the phone from.
Here's T-Mobile's unlock requirements: Unlock your mobile wireless device
Thank you for your reply,
Yes, I know that is really big favor I'm asking for. Basically I'm looking for someone who didn't change his phones very often and can help me with that.
Secondly, It's really impossible for me to get in contact with the original owner of the phone, the phone have been sold several times before it comes to me!
Don't they just verify -using the IMEI- if the sim card associated with the phone having no bills and so on?
Is there any possible solution in my case?
Last time I did a manual unlock (vs one via the app) it was on a phone I was given. The CSR said, "I don't see that phone being used on your account." I replied, "Wait a minute." and swapped my SIM into the phone and powered it up. After a moment she said, "Oh, now I see it. I'll e-mail you the unlock code." One of the current requirements is, "The account associated with the device must not be canceled, and be in good standing." I'm not sure how they enforce this. Does it mean "the original account" or "the current account"? I'm not sure.
All I can really suggest is that you poll your local friends and family to see if anyone uses T-Mobile and have them attempt to do the unlock and see what happens.
Actually you might want to try running the IMEI number here first IMEI Status Check | See if Your Phone Works on T-Mobile 4G Network
You will see that the device still has an outstanding balance, so even if you find an "unlock buddy" the phone doesn't qualify and only the original owner can settle the balance with TMO.