Net10 is a prepaid wireless provider in the United States. It is a subsidiary of Tracphone, another prepaid wireless service. Both companies are mobile virtual network operators (MVNO), which means they operate over another carrier's towers -- AT&T Wireless in this case. The phones that work on Net10 use SIM cards to register the phone number to the phone for activation. There are a few reasons for the error "unregistered SIM" appearing on the Net10 device.
The most common reason why a Net10 phone would say "unregistered SIM" is that the SIM card is not activated or was not activated properly. To fix this, simply go through the activation process again. By going to the Net10 website at www.net10.com, or calling 1-877-TEN-CENT with the phone's serial number, you can go through the activation process.
SIM has Expired
With Net10, the minutes expire and/or run out. When this happens, the phone will become deactivated. In this case, you must first purchase more minutes. Minutes can be purchased online, over the Net10 telephone number or in a retail outlet. Once the minutes are purchased, call 1-877-TEN-CENT to reactivate the phone and you will be able to use the phone immediately.
The SIM card itself may be damaged. There are many ways that a SIM card can become damaged, either by an electrical surge in the phone or physical damage, such as being bent or cut. When this happens, it has to be replaced, and if phone numbers are stored on the SIM card, they are likely lost. A replacement SIM can be obtained by buying another Net10 device.
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If you have tried to reactivate your device, add minutes and have determined that the SIM card hasn't been damaged, the last option is to replace the handset itself. If the device is defective, you will be able to return it to the store where you purchased it. If it is out of the return period, call Net10 customer service at 1-877-TEN-CENT and they will be able to help you troubleshoot your defective device.