Supplement 11 to ITU-T X-series Recommendations provides a technical framework based on a real-time blocking list (RBL) for countering voice over Internet protocol (VoIP) spam, which consists of four functional entities: a VoIP spam prevention system (VSPS), a VoIP spam prevention policy server (VSPPS), an RBL central system for VoIP spam prevention (VSP-RBL), and a user‑reputation system (URS). This supplement also specifies the functionalities, procedures, and interfaces of each functional entity for countering VoIP spam.