VoIP is a protocol designed to improve voice over IP and other networks based on packet switching. The birth of VoIP came as an alternative to the expensive public telephone network (PSTN) for voice transmission. Given QoS for VoIP, factors such as system capacity, frequency, packet delay, loss, and channel configurations are of paramount importance. These parameters with security issues, channel bandwidth allocation and reliability issue represent major challenges facing VoIP systems. To master Internet voice communication systems, a clear understanding of Internet Protocol (IP) is mandatory. The audio is transferred by the RTP protocol that is added in the IP packet. In this paper I will focus on the techniques used in VoIP systems and the challenges faced by these systems. As the voice transmission market in transition from traditional PSTN networks continues to VoIP, new technologies are being tested and tested to improve the quality of services provided by VoIP systems. The main focus is finding ways to deal with high traffic and booming demand for VoIP systems.