ETSI and 3rd Generation Partnership Project (3GPP) standards, such as GSM and LTE, define supplementary service codes that make it possible to query and set certain service parameters (e.g., call forwarding) directly from mobile devices.
See also GSM GSM services: Supplementary Services Vertical service code GSM USSD codes - Unstructured Supplementary Service Data: list of standard GSM codes for network and SIM related functions
External links 3GPP TS 22.030 GSM Feature Codes GSM Features That Aren't Widely Known What’s the difference between USSD, MMI and SS codes?