My guy friend is in China with his US cell phone and I’ve tried calling him from the US with my cell phone, but it won’t go through it’ll say his phone has restrictions that prevents, or its dead air. I know he let his phone company set his phone for international calling.
There isn’t much you can do here. When someone is traveling with their US cell phone abroad nothing changes in the way you dial their phone so make sure you don’t add any international codes. Other than this, if it doesn’t work the issue is on their end.