Session Initiation Protocol (SIP) has been widely used as the call setup protocol for multimedia communications such as Voice over IP (VoIP). The client-server architecture, however, exposes SIP servers to the risks of performance bottleneck and the single point of failure. Peer-to-Peer (P2P) SIP which is implemented by using the Distributed Hash Table (DHT)-based Chord is consequently proposed with the advantages of scalability and reliability by replacing centralized servers with the self-organized P2P network. Unfortunately, previous studies on P2P SIP have not yet considered the mobility issue which is an essential property of mobile Internet. While users roam in wireless networks, to ensure a stable DHT-based Chord introduces serious problems. In this paper, we propose a new unstructured P2P SIP system for mobile Internet telephony. An unstructured P2P network is less influenced by the churn property than a structured P2P network, and it is suitable for mobile environment. Moreover, the proposed unstructured P2P SIP network is constructed according to the relationships between SIP users and their friends. Therefore, SIP users can find their friends in the proposed P2P SIP network very easily. Also, the social network exhibits the small-world property, and an SIP user can locate non-friends in the proposed network within a small number of hops. Simulations results demonstrate that the proposed method introduces less maintenance overhead and achieves better performance than the conventional Chord-based approach.