How to fix USSD/MMI code connection problem or invalid code

Sometimes after install some applications the USSD code has errors like this Watch this video to fix it :Or read this A simple and effective way to fix Connection Problem or Invalid MMI Code on android phone is to add a comma at the end of the Prefix code. For example: If your Prefix code is *2904*7#, then adding the comma at the end would do just...

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