The so-called hidden codes are basically part of the Man-Machine Interface (MMI) schema. These are a bit different from the Unstructured Supplementary Service Data (USSD) codes, although both of them start with an asterisk (*) followed by digits that comprise of commands or data. Groups of digits may be separated by additional asterisks. The message is terminated with a hash symbol (#), but custom MMI codes may end with an asterisk as well.
While USSD codes are used to communicate with the mobile network operator’s servers for menu-based information services, mobile-money services, and location-based content services, the MMI Supplementary Service codes work completely offline.
Just like USSD, you have to dial the “hidden” codes on the stock dialer app to invoke the corresponding action.
Secret Codes !!!
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