It seems that your sim card is locked , so you have to unlock it , then only you have to use your mobile .............so get the sim unlock code for your Sony ericsson mobile through this site
www.simpleunlocking.com/sony-ericsson-j100i/rs16wp9 , it provides the step by step instructions and easy methods for unlocking your mobile from the network lock........at the same time to get the unlock code you have to give some information about the mobile to this site that is your IMEI number,network & country lock after that within a short period you can get the code ............
If you unlock your phone using the unlock code then your phone gets permanently unlocked....After that, you can use it with any other network's sim card...........
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