This solution applies to S-Series/K2/Cloud PBX and especially for the scenario when the incoming call is forwarded by the SIP trunk to the external number. Or we say to overcome the "Pinhole" difficulty for the RTP packets when PBX is behind a router.
; 0 indicates it is disabled. If it is an integer value greater than 0, it indicates it is enabled. Unit is second. Define a proper value when you use it.
Or see the article: Forwarding Call or Mobility Extension Has No Voice after Upgrade