only. Technical details Most GSM phones have USSD capability. USSD is generally associated with real-time or instant messaging services. There is no store-and-forward capability, as is typical of other short-message protocols like SMS. In other words, an SMSC is not present in the processing path. USSD Phase 1, as specified in GSM 02.90, only supports mobile-initiated ("pull") operations. In the core network, the message is delivered over MAP, USSD Phase 2, as specified in GSM 03.90. After entering a USSD code on a GSM handset, the reply from the GSM operator is displayed within a few seconds. Format A typical USSD message starts with an asterisk (*) followed by digits that comprise commands or data. Groups of digits may be separated by additional asterisks. The message is terminated with a hash symbol (#). USSD mode Mobile-initiated USSD/ PULL or USSD/ P2A when the user dials a code, e.g. *139# from a GSM mobile handset Network-initiated USSD/ PUSH or USSD/A2P when the user receives a push message from the network; primarily used for promotional services Man-Machine Interface The codes below are not USSD codes, these are the related Man-Machine Interface (MMI); they are standardized so they are the same on every GSM phone. They are interpreted by the handset first before a corresponding command (not the code itself) is sent to the network. These codes might not always work when using an AT interface; there are standard AT commands defined for each of these actions instead. BS is the type of bearer service, some valid values are: 11 for voice 13 for fax 16 for SMS (only valid for barring) 25 for data <leave blank> for all. T is number of seconds for the No Reply Condition Timer, default is 20 seconds if not specified. See also USSD Gateway Ultra Mobile Broadband Supplementary service codes SIM Application Toolkit References External links GSM 04.90 (ETSI EN 300 957, V7.0.1) Specification (USSD) Stage 3 – 3GPP 3GPP TS 22.030 What’s the difference between USSD, MMI and SS codes? Android USSD codes list (MMI code list) Mobile telecommunications standards 3GPP standards GSM standard