Natpass

NATPass is a NAT traversal product for VoIP networks based on SIP. The product is useful for both VoIP network operators that wish to offer a service to SIP analog adapters or IP phones behind residential routers or firewalls without having to configure each router or firewall to operate. NATPass uses several mechanisms for hole punching penetrate firewall and optimize the media flow. NATPass is available for Linux and is a commercially licensed product.