Class of Service (CoS) tagging is the correct answer because it is used to prioritize traffic at the data link layer, which is effective in a local area network environment like an office deploying IP phones. It allows devices to assign a priority level to packets so that network switches can provide the appropriate level of service. The other options, although related to network traffic management, do not specifically address the prioritization of voice traffic like CoS tagging does. Traffic shaping is more about controlling the overall traffic rate, and while WRED manages congestion, it does not inherently prioritize specific types of traffic. Ingress filtering primarily serves as a security mechanism to filter traffic based on predefined criteria and does not prioritize traffic.