For questions related to "Unstructured Supplementary Service Data" -- a sub-set of GSM codes, which are a subset of what you might know as "dialer codes" (note: the latter have their own tag, 'dialer-codes')

USSD or, fully pronounced, Unstructured Supplementary Service Data, are 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.

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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