Session Initiation Protocol, which can be applied to various networks for establishing, modifying, and terminating multimedia sessions, has received much attention in these years because of its wide applications. Because different users may belong to different networks and different networks may have different Key Generation Centers in identity-based networks, how to make different networks be compatible and how to make users belong to different Key Generation Centers can establish a secure Session Initiation Protocol session become two key problems. In this paper, we propose a new certificateless authenticated key agreement protocol for different networks to address the problems. The protocol is provably secure against a fully adaptive adversary in the random oracle model, provided that the underlying problem of Gap Bilinear Diffie–Hellman problem is hard. Moreover, our scheme has good performance compared with the certificateless authentication key agreement protocols using pairings. The trick proposed in this paper may give some help in making different networks be compatible. Copyright © 2012 John Wiley & Sons, Ltd.