IPhone problems with texting internationally?

Well, every iPhone I've had (iPhone 3, iPhone 4, iPhone 4s) got the same problem. When I was texting someone internationally (Australia - Canada) I can text for a while, then all of a sudden it stops receiving messages, and when I send them the person I'm texting says they never got the texts I sent. Why would it stop working all of a sudden? I'm not using iMessage because the other person has a different phone. Every time I text internationally, it stops working after a while. Any ideas what to do or what's wrong?

Did you download anything on them that could of made it not work or Mabey unlock your iPhone and change networks

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