This question was asked and different answers given, it's buried somewhere in this thread

It's definitely a YMMV situation.

If your primary use is domestic but you take periodic extended visits internationally, there's no issue. I travel to Asia for 2-3 weeks, 3-4 times a year and also did a 3 week vacation in Europe....never a problem.

Where you might get in trouble is if the majority of the time is spent abroad.....but even there, it's YMMV. My second son, on our family plan spent the last 8 months in Peru and used TMO as his primary data/text line. During that same time frame the Mrs. had two trips to Asia where she was gone 30 days each, and I had 2 trips for 3 weeks each. And our other son went to Japan for 2 weeks in same time frame.

Caveat....our family plan is over 10+ yrs old.