Depending on the frequencies used in a given country and if your SIM card has agreements in those countries, your coverage in certain countries while you are roaming internationally will vary. Nothing is worse than traveling with your home mobile phone only to discover it is inoperable. So why not acquire a global roaming SIM card that works in over a hundred nations? It might cost you hundreds, if not thousands of dollars, to answer and make one or two daily calls. Additionally, monitoring your emails may cost you several hundred bucks.