Grandstream Networks GXP 1620 User Manual page 7

5. Hover cursor over [Accounts]. Select [Account 1] and click [Network Settings].
Nat Traversal: Select [Keep Alive].
Once done click [Save and Apply]
6. Hover cursor over [Accounts]. Select [Account 1] and click [SIP Settings>Security
Settings].
Check SIP User ID for Incoming INVITE : [Yes]