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Step 1
Unlock the code on the cell phone. Nokia, LG and Samsung phones can be unlocked by entering a special code directly into the phone when you turn it on. Inputting *2767*2878# onto a Samsung X495, for example, will usually unlock it. Some phones, such as Motorola, Blackberry, Sony Ericsson and certain Samsung phones use a special program to unlock the phone. You can find these programs online (see Resources).
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Step 2
Remove the back cover of the phone. Make sure that you put it in a safe place. You will need to replace it later. On Palm Treo phones, you can skip to Step 4 because the SIM card slot is on top of the phone near the antenna.
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Step 3
Take the battery out of the phone. Beneath the battery, you will notice a slot for a SIM card. In some phones, you will have to open a hinged protector. This is common on some Motorola and Nokia phones.
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Step 4
Slide your Cingular or AT&T SIM card into the slot. Make sure that you put it in the correct way. The gold chip should be against the phone. One corner of the SIM card is cut off and that should be toward the top of the phone in most models.
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Step 5
Replace the battery and then replace the back cover. Once you have done this, turn the phone on and try to make a call. If you see a message that the SIM is unavailable, make sure that the SIM card is in the phone correctly. If it is and you still get an error message, you need to try a different unlock code or send your phone to a professional to unlock it.
