If your Android phone shows "Connection Problem or Invalid MMI Code" when you dial something like *#21# or *135#, don't panic. This is one of those errors that looks scarier than it actually is. Most ...

If a code returns "Connection problem or invalid MMI code," your phone, Android version, or carrier does not support it. Phone and Software Information Codes These read-only codes reveal hardware and ...

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This is the fastest fix because it forces your phone to completely disconnect from and re-register with your carrier’s network. A surprising number of MMI code errors come from a temporary network ...