USSD

USSD (or Unstructured Supplementary Service Data) is a communication protocol available on every GSM-enabled mobile device. It is a session-based text communication without a store-and-forward mechanism (unlike SMS) that is practical for interactive communication, such as banking or education. USSD messages can have up to 182 alphanumeric characters and the time it takes from a request to a response is 2 seconds while it takes 6 seconds for an SMS to reach a mobile phone.