Present enterprise communications systems and applications provide integration of voice and data services. Still, proprietary technology and architecture discrepancies make it difficult to implement and deploy new converged communications services. The Session Initiation Protocol (SIP) brings user centric Internet Telephony. This model provides new converged communications services based on presence, mobility and user preferences. SIP technology promotes distributed architecture models, and seamless integration with IP technology. SIP is an open extensible Voice over IP (VoIP) protocol and is a framework for building voice and data communication applications. This article presents what SIP brings to enterprise communication applications and how customers can manage a smooth transition to SIP based services with the example of the OmniPCXIP communication server.