Extending the university campus Public Switched Telephony Network (PSTN) network and adding new capabilities to it need a new Public Branch Exchange (PBX) system with high cost. The extension can be achieved using voice over IP (VoIP) on the university campus data network, which reduces the costs and increases flexibility in providing new value added services. Obviously the VoIP network should be interoperable with the existing PSTN network. To implement this solution design choices should be made to determine VoIP signaling protocols, servers, and signaling protocols between the two networks using gateways or softswitches. Issues such as reliability, redundancy, accessibility, interoperability with other solutions, QoS, security, and extensibility should also be addressed. In this thesis, first we review possible commercial solutions provided by companies active in this market. Since these solutions are usually expensive, we then propose a solution based on free and open source softwares. The proposed solution is based on SIP protocol for signaling and uses SER server which is an open source solution for SIP proxy server as well as SIP redirect server and the registrar. For communication with the data network beyond the university campus network and the Internet, packets should pass the NAT. Signaling protocols such as SIP have problems in passing Network Address Translator (NAT). RTP Proxy is a suitable way to solve the NAT problem but it requires that the RTP packets are also transferred through the server. We propose a solution in which the RTP packets do not pass from the server.