I was gifted a new Samsung Note 5 from my relative ( originally brought from Samsung Store). I activated the T-Mobile prepaid card and begin using it. However when I being my international travel, they wouldnt unlock my mobile. I fulfilled their conditions however, they still kept saying that this is not a t-mobile phone. I confirmed this with Samsung Mobile support and they are ready to vouch for it. However T-Mobile still does not unlock my phone.
The Mobile is a clear T-Mobile phone ( starts with T_Mobile logo, backdrop, has built in T_mobile "Device Unlock" app. When I insert a non T-Mobile sim, it says the carrier is locked and asks me to contact customer service. There is a communication gap between Samsung and T-Mobile for which I am stressed out making numerous International calls to each and neither of them want to own the mobile which is under warranty.
That's a complex one. I suppose it goes without saying that you should have confirmed the phone's unlock status before going abroad. I think this one may have to wait until you get back to the US. You might want to find out if your relative saved the receipt when from when they bought the phone. In the mean time, if you're using your T-Mobile SIM, calls made to T-Mobile's international support number (+1-505-998-3793) are free.