Anyone know if my sister in law who is here from Germany with her iPhone 6 that says t mobile on it can use a t mobile sim from the U.S. The model number of her phone is MG4J2ZD/A. Would the phone likely be locked? Okay...I should add I just put my Att sim in her phone and it worked. So I'm wondering if I should stick with Att or get a t mobile one? She needs to stream some YouTube for her ailing father so we would prefer T mobile for pricing. Any help would be greatly appreciated!