-Turn off phone, open itunes
-Hold home botton and insert usb cable
-Say "ok" and "restore" iphone (you will lose all user contents)
-If phone factory unlocked put any sim card to actvate it.
-If not factory unlocked, put original carrier sim, or jailbreack iphone to actvate it.
PS: If not factory unlocked, it will became locked again and you need to use a gevey sim to get signal.
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