How do I turn off International calling entirely on my T-Mobile phone line?

I had the same issue, and needed to disable outgoing international calls. I called in and the representative (Mariana) was very helpful and able to take care of it successfully. She was also able to apply the block only on individual lines which was helpful. Also note that it applies to all outgoing international calls whether you're in the US or traveling abroad. So you'll want to remove the block when traveling abroad so that you can make calls (at $0.25/min).