Chapter 22 VoIP Solutions: SIP : FortiGate VoIP solutions: SIP : Hosted NAT traversal : Restricting the RTP source IP
  
Restricting the RTP source IP
Use the following command in a VoIP profile to restrict the RTP source IP to be the same as the SIP source IP when hosted NAT traversal is enabled.
config voip profile
edit VoIP_HNT
config sip
set hosted-nat-traversal enable
set hnt-restrict-source-ip enable
end
end