Abstract:Undoubted every organization's heart is it's phone system. The old phone systems couldn't perform any method to make phone center and voice transmission intelligent on network and they had determinate abilities. Meantime Voice over Internet Protocol (VoIP) introduced itself to the world and performed a lot of abilities for clients like voice transmission on network. Many companies Investment on voip systems and implemented their methods on software and hardware packages. But between them a different production had designed and Performanced by Mark Spenser from Digium Company in 1992 which named Asterisk. Asterisk's increasing popularity's reason was its open code programs and its flexibility. VoIP systems such as Asterisk use voice transmission protocols to transfer voice over network. One of the voice transmission protocols is Session Initiation Protocol (SIP), which is one of the Asterisk's voice transmission protocols. The first and the most important point in voice transmission over network is security. Security can be divided to two parts as inscrutability of invaders to network and coding transmitted voices over network to prevention of illegal listening. In this project at first we tried to introduce Asterisk phone system's structure and Session Initiation Protocol (SIP) and then scrutiny method of Invader's dominance to this protocol and Performance modern methods to prevent hacker's dominance and also coding voice packages to Obscure them in transmission way.