USSD stands for Unstructured Supplementary Service Data and is a GSM service that allows a phone to create a session with a software application (typically in the Mobile Operators network). Unlike SMS, USSD is a session based protocol which allows for real time interaction between the software and the end user. The most common use of USSD is by operators themselves to enable end users to check their balance, re-provision settings and check settings e.g. getting your IMEI code can be done by dialling *#06#. Note that this should not be confused with ** codes, which are actual dial-able short numbers.
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