Abstract : The opportunity to use data networks for telephone conversations is appealing. The technology to do this is commonly known as Voice over Internet Protocol (VoIP) or IP telephony, and has become widely available during the past few years. As part of our work concerning the deploying of Unified Communications (UC) for Mosul University network, this paper presents simulation for SIP-Based VoIP as one technology of UC technologies. Voice telephony is considered as real-time communication service and time sensitive service. To ensure if the University IP network is ready and capable for this new type of traffic before adding any new components, Mosul University IP network will be simulated using OPNET network simulation software then the new VoIP service will be added to the University network. Network performance and voice quality are tested after adding the VoIP for different codec types. The results show that Mosul University IP network is capable to implement the VoIP. Keywords: VoIP, Session Initiation Protocol (SIP), IP Telephony, Simulation, OPNET, Analysis.