USSD is commonly used by prepaid GSM cellular phones to query the available balance. The vendor's "check balance" application hides the details of the USSD protocol from the user. On some pay as you go networks, such as Tesco Mobile, once a user performs an action that costs money, the user sees a USSD message with his or her new balance. USSD can also be used to refill the balance on the user's SIM card and to deliver one-time passwords or PIN codes.