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The SIM is compatible with any unlocked GSM cell phone with the appropriate GSM bands. Like many international SIM card services, the National Geographic SIM card supports multi-IMSI technology, allowing for both a US and UK based phone number on one SIM card. Unlike other services, however, both numbers are always active on the National Geographic SIM and do not require a monthly fee to maintain.
Incoming calls on the UK number are free in more than 70 countries, though there is currently a “per minute” charge to all calls made to the U.S.-based number. Like many of its counterparts, the National Geographic SIM service also offers auto recharging features so users don’t have to manually top up their accounts and is prepaid, so expensive overage charges are a thing of the past.