BACKGROUND

The Global System for Mobile Communications (GSM) is a standard for digital cellular telephone networks. Although GSM is a second-generation technology, GSM functionality continues to be widely supported in subsequent generation technologies, such as the Universal Mobile Telecommunications System (UMTS) and Long-Term Evolution (LTE). Unstructured Supplementary Service Data (USSD) is one communications protocol used by GSM devices to communicate with systems of the mobile network operator. In one example, a USSD message may start with an asterisk (*) and be followed by digits that contain commands or data. Groups of digits may be separated by additional asterisks. The message is terminated with a hash symbol (#). USSD messages may contain up to 182 alphanumeric characters. USSD messages may be initiated by the mobile device or by the mobile network operator.

