The rapid growth of the SIP implementations clearly shows that the classical PSTN network will be exchanged by internet telephony. All Service Providers (SP) are currently deploying SIP infrastructure for offering the next generation networks. The implementations and network concepts used by the SP are different then the original designed in SIP standard. The main SP reasons are protecting their network, high number of customers, service quality quarantine and duties to the executive power. The security aspects in VoIP are generally secondary and planned for the coming years, since at first stage the target is launching the service on time. In this paper we are showing the current provider?s SIP implementations and giving the motivation for this structure. Then we are analyzing the possibilities for securing the communication involving the existing standards. There are many security layers and objectives in the SIP networks to be considered. Some of them are practically not possible in the SP network and other are not reasonable. In this paper we giving the understanding, what can be done and how the different security mechanisms are interacting. At the end recommendation for protecting the SIP communication are made.