USSD is used to request information and pull it down to the mobile phone. Users enter a specific USSD code to request information that they receive back in the form of a short message.
Available information includes finance, travel and subscriber account details.
By entering a USSD string, Vodafone customers can receive back from the network their GSM telephone number. Of course, USSD is an ideal bearer for roaming services since it uses the home network's infrastructure. When you make a call, a recorded voice announcement tells you how long your remaining subscription will let you call that number for.