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"Consideration
Even when a customer turns off his cellphone while in a foreign country, he can incur roaming charges. When a customer turns on his device in another country, the local carrier will register the device on its network. This means that even if the customer keeps his phone turned off or chooses not to answer calls, the customer's home carrier will still route calls to the local carrier, which then forwards the calls back to the home carrier. The local carrier will often charge a per minute rate to route calls. If the caller leaves voice mail, the customer may have to pay for it."
Read more: What Are Cellphone Roaming Charges? | eHow.com
http://www.ehow.com/info_8370495_cellphone-roaming-charges.html#ixzz2NEgc1eMt