One of the most preferred communication tools in the telecommunication world is telephony. Telephone exchanges that do not use the functioning Internet Protocol (IP) technology are beginning to lose importance. Instead, exchange systems that provide communication over IP have begun to be used. Although, Voice over IP (VoIP) technology provides great advantages over traditional telephone exchanges as enabling voice transmission over IP, security and safety concerns are seen as critical issues for VoIP technology and devices or software applications using this technology. VoIP security and safety is directly related with Internet security, since VoIP technology uses Internet infrastructure during communication and the data is transmitted between devices as IP packets. In this study, old-fashioned telephone exchange systems and systems using IP technology are compared at first glance. Secondly, threats, security and safety issues related to VoIP are addressed and studies consisting of identification of threats, identification of security measures, testing of these security measures, situations threaten VoIP security and vulnerabilities of VoIP technology are examined. Threat categorizations of Voice Over IP Security Alliance (VoIPSA) and the Internet Engineering Task Force (IETF) are examined. Papers on VoIP security and safety are classified according to VoIPSA and IETF threat taxonomy. Lastly, possible and novel counter-measures to those security and safety threats are proposed.