What happens if one user calls another, and then, after the call has been set up, sends a SIP BYE message to stop the billing while continuing to send RTP packets to the other user? If one end of the call is a trusted device, such as a PSTN gateway, it will pay attention to the BYE and tear down the call. If neither user is trusted, the call may stay up, but the PEP in the network will remove the QoS for the call and it will shift to “best effort” quality. Some networks, particularly those running 3G wireless, may be configured to stop packet transport if QoS has been removed.
A new technology being used to fight toll fraud is the Back-to-Back User Agent (B2BUA) that offers the possibility of prepaid calling applications. For more information, see the definition of the B2BUA offered near the end of Chapter 10.