So had a nightmare of a time with my new Samsung Galaxy S6. The new process of unlocking which is likely to pave the way for the unlocking of all phones on tmobile a day other US carrier networks requires unlocking via an app that resides on the phone. The old method of unlocking which required an unlock code supplied by the carrier will likely be discontinued entirely.
In my case it was one thing after another with tmobile and meeting their requirements even though at the time of purchase of the phone I was told the phone was unlocked already for certain.
I managed to unlock the phone in the end but one thing that the tech support at tmobile does not tell you is that your device cannot be encrypted in order to unlock using their app. After decrypting the device and running the app is was able to unlock the phone. So anyone else out there who has a locked Samsung or other phone who are experiencing problems with unlocking their phones even though all conditions are met this may be the problem for you too.
Since unlocking my Samsung Galaxy S6 using the unlock app on my phone I have been able to use the phone without an issue here in St. Lucia where I live on the Digicel network without any issues.