Grandstream Networks GXV3240D Administration Manual page 39

Proxy-Require
Firmware Version 1.0.1.54
If set to "STUN" and STUN server is configured, the phone will route
according to the STUN server. If NAT type is Full Cone, Restricted Cone
or Port-Restricted Cone, the phone will try to use public IP addresses
and port number in all the SIP&SDP messages. The phone will send
empty SDP packet to the SIP server periodically to keep the NAT port
open if it is configured to be "Keep-alive". Configure this to be "NAT NO"
if an outbound proxy is used. "STUN" cannot be used if the detected
NAT is symmetric NAT. Set this to "VPN" if OpenVPN is used.
A SIP Extension to notify the SIP server that the phone is behind a
NAT/Firewall. Do not configure this parameter unless this feature is
supported on the SIP server.
GXV3240D Administration Guide Page 39 of 83