The Session Initiation Protocol (SIP) is one popular signaling protocol used in many collaborative applications like VoIP, instant messaging and presence. In this paper, we evaluate one well-known SIP proxy server (i.e. OpenSER) on two multi-core platforms: SUN Niagara and Intel Clovertown, which are installed with Solaris OS and Linux OS respectively. Through the evaluation, we identify three factors that determine the performance scalability of OpenSER server. One is inside the OSes: overhead from the coarse-grained locks used in the UDP socket layer. Others are specific to the multi-process programming model: 1.overhead caused by passing socket descriptors among processes; 2. overhead brought by sharing transaction objects among processes. To remedy these problems, we propose several incremental optimizations, including out-of-box dispatcher, light-weight connection dispatcher and dataset partition, and achieve significant improvements: for UDP and TCP transport, on SUN Niagara, speedup (ideal is 8) are improved from 1.5 to 5.8 and from 2.2 to 6.2, respectively; on Intel Clovertown, speedup (ideal is 8) are improved from 1.2 to 3.1 and from 2.6 to 4.8, respectively.