Recent research activities have proposed using a peer-to-peer network for user registration and user location in session initiation protocol (SIP)-based voice-over-IP (VoIP) networks. The main motivation for peer-to-peer (P2P) SIP is higher robustness as well as easy configuration and maintenance (compared to client-server SIP). However, these advantages come at the price of security. In this article the security challenges of using a P2P network as a substrate for SIP communication are explored. After a short introduction to SIP and structured P2P networks, the different approaches that have been proposed for using P2P technology in conjunction with SIP are compared. We focus on design issues that affect security in order to identify the security implications of using a structured overlay network for SIP registration and location lookup. Finally, possible solutions for securing P2P SIP are examined and explicit suggestions on how to improve the security of P2P SIP are given