The more precise term is Supplementary Service Codes, and they consist of numbers enclosed by # and * characters. This name can refer to:

  • or, fully pronounced, Unstructured Supplementary Service Data: used by GSM cellular telephones to communicate with the service provider's computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network.
  • Supplementary Service Codes themselves: make it possible to query and set certain service parameters (e.g., call forwarding) directly from mobile devices.
  • Manufacturer specific service codes (to invoke things like hardware tests, running debug routines, or enable service menus, intended for the use by the manufacturer's service personal)

USSD codes usually follow the pattern *1nn*<Parameter># (where *<Parameter> is an optional part), with nn being two additional digits. The other two groups use more complex code patterns, often something like #*#*n{2..8}*#*# (i.e. two to eight digits enclosed).

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