You cannot put a prepaid phone into "global roam" mode. Michelle is correct about using a prepaid phone that has been unlocked (most prepaids are locked into a certain carrier(ie...Telsta) for a peroid of time as they subsidise the cost of the handset) on a Thai sim card, but this is not Global roaming. GR refers to using your phone with your sim card on an overseas network who bill your carrier, who susequently bills you. THe problem with this is that you will get charged for incoming as well as outgoing calls at a amazingly high rate, but you keep your phone number. This is only practical for business travellers, it is much cheaper to get a Thai sim card and use that, as their call rates are incredibly cheap...even the lowly paid hotel workers all have phones.

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