In this paper we present the results of the experimental analysis of the transmission of voice over secure communication links implementing IPsec. Critical parameters characterizing the real-time transmission of voice over an IPsec-ured Internet connection, as well as techniques that could be adopted to overcome some of the limitations of VoIPsec (Voice over IPsec), are presented. Our results show that the effective bandwidth can be reduced up to 50% with respect to VoIP in case of VoIPsec. Furthermore, we show that the cryptographic engine may hurt the perfomance of voice traffic because of the impossibility to schedule the access to it in order to prioritize traffic. We present an efficient solution for packet header compression, which we call cIPsec, for VoIPsec traffic. Simulation results show that the proposed compression scheme significantly reduces the overhead of packet headers, thus increasing the effective bandwidth used by the transmission. In particular, when cIPsec is adopted, the average packet size is only 2% bigger than in the plain case (VoIP), which makes VoIPsec and VoIP equivalent from the bandwidth usage point of view.