USSD Code List
A USSD code is a short string. The mobile user dials the USSD code for a service from a mobile operator. The routing of USSD codes is always towards the home network of the serving mobile operator.
Each mobile operator has its list of USSD codes.
Each code has a specific meaning in-home network. If a mobile user dials a code, which is not in the list of codes provided by mobile service providers, the user gets an invalid MMI error or a USSD session gets a timeout. As USSD codes have significance for the serving operator only, two or more mobile operators have a common code list.
When a code is dialed on the handset, the roaming VLR forwards the USSD code as a USSD string in process USSD protocol message to the home network. The address of the home network is determined by the HLR number received in the update location response.