Lawful Interception is one of the main provisions needed for the security agencies to monitor a suspect or deal with criminal activities. The implementation of this was easy in the traditional telephone networks, because of its centralized administration. However this has become a tedious job with the emergence of VoIP technologies where voice traffic is carried over IP. In this paper, we discuss the challenges and propose a monitoring architecture for Lawful Interception on VoIP networks based on SIP. We also explain how this can be achieved from the existing protocols and solutions like NETCONF and RBAC model. Finally, we present our prototype implementation of the Lawful Interception, the limitations and the difficulties that we found.