Yes it seems once you finally get on once, you get on all the time without issue. But a quick search of the Inter Webs shows tons of people with the same issue, indeed my wife couldn't get on with her iPhone4 after I got on with my iPhone5 at first I thought it was an iPhone issue as loads of people with the same issue had iPhones but clearly since I got on with mine it must be something else.
But I think it's valid to say, if you really need to stay in contact, get a sim that works in the USA as a back up.