Today, the session initiation protocol (SIP) is the predominant protocol for Voice-over-IP (VoIP) signalling. The intention of this paper is to present an overview of VoIP security issues both current and future – focusing on SIP. We start by presenting some fundamental differences between VoIP and the public switched telephone network (PSTN). We then look at specific problems for SIP signalling that arise from these differences. We summarize current activities regarding SIP security, including recent developments in the research community and standardization efforts within the IETF. Finally, the paper will give an outlook on security issues in future VoIP scenarios. Specifically, we present a short security analysis of using SIP in a peer-to-peer setting (P2P-SIP).