Yes the DVG-2001S can work behind an ADSL router or InternetGateway device running NAT.  This is made possible usingSTUN. 

Simple Traversal of UDP over NAT (STUN) ? is a protocol thatenables a VoIP device, such as the DVG-2001S or an IP phone, todetect the presence and type of NAT behind which the phone isplaced. The DVG-2001S supports STUN and can intelligently modifythe private IP address and port in its SIP/SDP message by using theNAT mapped public IP address and port through a series of STUNqueries against a STUN server located on the public Internet. Thiswill allow SIP signaling and RTP media to successfully traverse aNAT without requiring any configuration changes on the NAT.