Protecting the anonymity of users in Voice over IP (VoIP) systems is an important enabler for many services, e.g. counseling helplines. Systems based on mixes have been proposed to protect anonymity in such scenarios and interactive applications in general. However, existing systems are either vulnerable to passive and active timing analysis, unsuitable for VoIP due to bad quality of service (QoS) properties, or not scalable due to intense usage of cover traffic. Therefore, we revisit a recently proposed approach of using a fully meshed mix overlay of padded links, i.e. sending packets with a fixed rate between all pairs of mixes. While using padded links is a promising approach, using a full mesh does not scale. By solving mixed integer linear programs (MILPs), we construct topologies that maximize anonymity and yet are subject to restricted node degrees. We evaluate our approach using a latency dataset from the live Tor network and a simulation driven evaluation of the topologies. The results show that we can significantly reduce the call blocking rate without degrading anonymity by restricting the node degrees. This implies that open systems like Tor may achieve QoS properties suitable for VoIP applications.