I’m sure most Android users have at some point received a message saying “Connection problem or invalid MMI code”. This can sometimes be so frustrating and so painful to deal with. When the invalid MMI error code appears, it simply means that calls and text messages cannot be sent until that particular problem has been resolved.
There are several problems for which the error message “Connection problem” or “Invalid MMI code” appears. The main reason, however, is probably due to service provider issues or issues related to SIM card authentication on your smartphone.