VOIP and Internet multimedia system technologies are rapidly being used  by consumers, government, enterprises and militaries for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as Internet. These technologies offer more features than traditional PSTN (Public Switched Telephone Network). VoIP provide more flexibility and cost is also reduced in VoIP than PSTN[2]. Also, VOIP systems are more complex in architecture, protocols and implementation terms with increase in the potential of misuse. This paper presents an overview of VoIP system and some of its security issues to provide a direction towards the future research in similar technologies. This paper also gives a brief overview of SIP, one of the most important technology of VoIP.