SUMMARYA secure architecture is proposed in the home/office networking environment with a special appliance, called Building Security Gateway (BSG), for cheap SIP devices and unsupported soft user agents (UAs) performing security functions successfully, such as construction of secure sessions and encrypting/decrypting media. Therefore, we apply BSG procedure to VoIP for getting secure session. Furthermore, VoIP call monitoring and intercepting functions are also implemented in BSG. Secure session monitoring is supported in this architecture. A prototype BSG is built in a home/office network, and BSG-based session mobility cases between UAs are examined and the protocol primitives for security and call intercepting are verified.