Many android users will have meet this message on your android phone one way or another. This usually happens when u press the *,# or + and the phone button to made the call.

After u press the *,# follow by some numbers, it shows USSD Code is running and later the Connection problem or invalid MMI Code will appear. You have enter a code the system does not understand. But if u have press *#06#, it will display the IMEI Code. There are many special codes that start with * or # and follow by numbers and pressing the SEND or Phone icons.

If you start up the phone and this error prompts, it means the sim card or your phone system is automatically sending a special code to the cell phone company but not recongnized by the cell phone company system.

What is a MMI Code?
An MMI Code is a sequence of digits that is entered into your phone to make it perform some special action. These codes start and end with a * or # so they don't get confused with phone numbers you might want to dial. They're usually only valid when you're not on a call, so if you key one in while connected to your cellphone company's IVR system they won't be recognized. Some MMI Codes are dealt with by your phone – for example, there's usually one you can key in to find out what software version your phone has installed. Most MMI Codes are sent to your Cellphone company as they are used to turn features on and off (Like call forwarding, call waiting, etc).

Why does my phone say "MMI Code Failed"?
This will usually happen because your Cellphone company has decided that you aren't allowed to forward your calls. It could be because you are on a "plan" that doesn't have the forwarding feature (pre-pay/flex-pay plans usually restrict this feature). Unfortunately there is nothing we can do to 'bypass' this error – if you believe your plan should allow you to use this feature you will need to call your cellphone company and ask them to (a) let you control it from your phone, or (b) set the forwarding for you.
Note: If you go for option (b) make very sure that they only turn on "Conditional" forwarding (this only forwards Busy/No-Answer/Unavailable calls). Sometimes they'll accidentally turn on "Unconditional" forwarding (this forwards ALL calls!).

Credits from youmail.com