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If you are an Android smartphone user, many times you have come across a message like 'Connection problem or invalid MMI Code' on dailing a USSD code.
It's a serious problem which affects the phones resulting in imposing some restrictions while sending and receiving text messages and calls, checking account balance, recharging mobile number and so on.
Though the error is generally seen in the dual SIM mobile phones, it may sometimes also occurs in single SIM phones. It mostly occurs either due to issues with the carrier or because of SIM authentication problems.
To get rid out of this issue, we are 5 effective yet simple methods which will solve the 'Connection Problem or Invalid MMI Code' problem.
Method 1. Enter Safe Mode
Safe mode temporarily disables all pre-installed applications and services that came with the phone. It helps to find out the culprit app that may be interfering with the network operation of the phone. To enter Safe Mode, turn off the phone and hold the power button. While holding the power button, keep tapping on the menu button until you see the safe mode option at the bottom left of the screen.
Method 2. Modify Prefix Code
An alternative way to solve your connection problem is quite simple. You just need to add a comma (,) at the end of the prefix code (*135#). This addition forces the operation to look for the error. You can also add '+' (plus) after your prefix code. It also works the same way like a comma.
Method 3. Activate Radio to Turn on IMS over SMS
This is another way to solve connection issue. In this method, you need to enter the information mode and activate radio to turn on IMS over SMS. Here are the steps to be followed: dial *#*#4636#*#* in the dialer>navigate to Phone Information>click on 'Run Ping Test'>Once the ping test is performed>click on 'Turn Off Radio'>now click on "Turn on IMS over SMS'. Finally, Reboot your phone. This must solve your issue.
Method 4. Restart Phone Several Times
Simply turn your phone on and navigate to network settings. Then hold the power and the home button together until your phone turns off. Then turn it back on.
Method 5. Select Network Operator Manually
Navigate to Settings>Network Connection>Mobile Networks>Network Operators. You will see available networks list. From there select your service provider and repeat the process 4-5 times until it actually starts working again.