Grandstream Networks GXV3275 Administration Manual page 52

If the firewall and the SIP device behind the firewall are both able to
use uPNP, it can be set to "uPNP". The both parties will negotiate to
use which port to allow SIP through.
The default setting is "Keep-alive".
It is used to add the Proxy-Required header in the SIP message. It is
used to indicate proxy-sensitive features that must be supported by the
Proxy-Require
proxy. Do not configure this parameter unless this feature is supported
on the SIP server.
Firmware Version 1.0.3.46 GXV3275 Administration Guide Page 50 of 117