Been a T-Mobile customer for 2+ years now. We pay $110 a month for 2 phones, and have unlimited data, calling, and have overseas calling.
Calls are made extremely often to Asia, and rarely have any problems....I live in Southern California. The farthest north the phone has been is Lake Louise in Canada, and points between.
Been up the Inside Passage of Alaska, with the attendant stops in Ports of Call, no problems.
Been as far east as Mount Rushmore, by bus...no problems.
One phone went to Tennessee, calls went perfectly.
Before my wife was with ATT, and I saw that her monthly bills were often $200 because of the overseas calls. ATT wanted to nickle and dime every chance they got.
I agree with what others have said....it all depends on your location at the time of making the calls. You might have T-Mobile coverage, but it may not be 4G.