Unlocked the phone

You must meet the following criteria to request an unlock thru T-Mobile US.  If you meet the requirements the procedure on how to request it and the requirements are provided here:

Unlock your mobile wireless device

The biggest is you must be a former and active T-Mobile US customer or they will not unlock the device.  If you are not the original owner and not a former T-Mobile US customer you will need to use a 3rd party unlocking service (you will have to Google to find one).  Before paying for that I would make sure the device is not blacklisted (i.e. listed as stolen or still has a financial obligation against it) here otherwise even unlocked it won't work on a lot of networks as they all share blacklisted IMEI numbers; IMEI Status Check | See if Your Phone Works on T-Mobile 4G Network