Question: How do I determine the NAT type?
Answer:

NAT traversal can influence registration if the GXV3140 is on a LAN and needs to register to a SIP server on a public IP (you must have NAT traversal enabled). Please select the NAT traversal method according to your network environment. If you are unsure, it is recommended to select “Auto” to enable the automatic NAT traversal configuration feature.

GXV3140 Screenshot NAT

NAT traversal is not required if the phone and SIP server are on the same LAN or the phone has a public IP address.

There is also an easy way to check if the phone needs NAT traversal or not. After the phone is installed, dial 0 on the phone. If you are able to see a welcome video with a rotating globe on the LCD screen then there is no problem with the NAT traversal. If you do not see this, this means that external UDP packets cannot be forwarded to this phone.