Simple Traversal of UDP over NAT (STUN) ? is a protocol that enables a VoIP device, such as the DVG-2001S or an IP phone, to detect the presence and type of NAT behind which the phone is placed. The DVG-2001S supports STUN and can intelligently modify the private IP address and port in its SIP/SDP message by using the NAT mapped public IP address and port through a series of STUN queries against a STUN server located on the public Internet. This will allow SIP signalling and RTP media to successfully traverse a NAT without requiring any configuration changes on the NAT.

Below is a list of Public STUN servers

• stun.ekiga.net
• stun.fwdnet.net
• stun.fwd.org (no DNS SRV record)
• stun01.sipphone.com (no DNS SRV record)
• stun.softjoys.com (no DNS SRV record)
• stun.voipbuster.com (no DNS SRV record)
• stun.voxgratia.org (no DNS SRV record)
• stun.xten.com
• stunserver.org see their usage policy