MMI stands for “Man-Machine Interface” and refers to the series of numbers or text entered into your phone to perform actions. Theses MMI codes typically start with an asterisk and end with the pound symbol. This prevents the phone from interpreting the information entered between these symbols as a phone number you are trying to call. If you’re on a call, you cannot access MMI codes. Many of these codes are programmed into your phone by the phone company. Your user manual will define those for you.