A processor architecture for processing data packets representing voice over Internet Protocol (VoIP) calls in a packet-switched network is disclosed. According to an embodiment, a VoIP processor executes a voice packet processing operating system that is configured to monitor or manipulate the packets at an IP layer, media layer and signaling layer of the call. The VoIP processor includes a plurality of independently callable primitive software functions that carry out low-level VoIP packet processing functions. The VoIP processor executes one or more application programs that selectively call one or more of the primitive software functions and are independent of any underlying protocols of the existing network, thereby isolating the application programs from low-level processing details. Further, techniques are described for modifying characteristics of VoIP traffic for the purpose of monitoring and directing the VoIP traffic through a network. The techniques include extracting information associated with the VoIP traffic and using the information for the purpose of controlling access, for fraud detection, for billing, for enforcing policy decisions, for protection against denial of service attacks, for lawful interception, for service selection, and other applications.