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Dual SIM iPhone 4 cover
What the kit does is that it replaces the iPhone 4′s microSIM tray with an external dual SIM adapter, which will give the iPhone 4 the capabilities of using two phone accounts from two different carriers. Unfortunately, unlike true dual SIM enabled phones, this dual SIM cover will only allow you to use one SIM card at a time, and would also require you to go to the settings whenever you wish to swap to the other SIM.
It seems to be rather clumsy, but I’m guessing for those who require two numbers on one device without having to stop and swap SIM cards will most probably appreciate this. The dual SIM iPhone 4 cover is currently retailing for $70 (49.90 Euros) at Vaveliero’s website if you’re interested in getting your hands on it.