USSD (Unstructured Supplementary Service Data) is a Global System for Mobile(GSM) communication technology that is used to send text between a mobile phone and an application program in the network. … USSD is similar to Short Messaging Service (SMS), but, unlike SMS, USSD transactions occur during the session only.
USSD works both on smart phones and feature (simple) phones. USSD code can either be Prepaid (end user meets the cost) or Post Paid (code owner meets the Cost).