Download the go sms app from the playstore and you can edit the apn setting but most of the time it just works and updates them for you . You can now use the native texting app to send pics and not go sms ,just keep it on the phone. Alternately(if your not a phone teck) and you need your msl its very easy to get on the s3 just follow these instructions for root (link below) and download get msl from playstore after towel root make sure you download su from playstore and install and confirm you have su rights before you run get my msl
[Only registered and activated users can see links. ]
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