The Session Initiation Protocol (SIP) is the de facto standard for multimedia multiparty sessions signalling in Next Generation Networks (NGN). It is the basis of a wide range of IP multimedia services. SIP relies on centralized servers, as per its standard specifications and current usage. However, research has recently begun on the integration of Peer-to-Peer (P2P) principles to SIP to capitalize on the benefits of decentralization. This paper contributes to this research by proposing a novel architecture for P2P SIP. The architecture is an overlay composed of a set of self-organized proxies and distributed registrars. Unlike the architectures proposed to date, it does not extend SIP messages and is P2P middleware independent. This eases implementation, interoperability with legacy, and ensures portability. We have prototyped it using JXTA. JXTA is a P2P middleware that can be deployed on devices ranging from cellular phones to high-end servers and including laptops. Requirements are derived and the related work is discussed. The architecture is presented and the implementation, including the prototype, is described.