USSD Code List
A USSD code is the short string. The mobile user dials the USSD code for service from a mobile operator. The routing of codes always towards the home network of a mobile operator. Each mobile operators have its own 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 code provided by mobile service providers, the user gets invalid MMI error or a USSD session gets a timeout. As USSD codes have significance for each operator only, two or more mobile operators have common code list.
When a code is dialled on the handset, the roaming VLR forwards the USSD code as 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 update location response.