TekSIP is a freeware SIP Registrar and Stateless SIP Proxy for Windows. You're free to use it for both commercial and personal use. We only ask you to provide feedback. TekSIP complies with RFC 3261, RFC 3263, RFC 3311, RFC 3581 and RFC 3891. It supports NAT traversal. Please see technical details about NAT traversal in Readme.txt You can select IP address to be listened and default SIP endpoint for outgouing calls. You can also log session details into a log file and monitor active registrations and sessions in real-time.