MiVoice 400 SIP trunking one-way audio

In this scenario we connected an NEC SL1000 to a Mitel MiVoice 400 (A470) via SIP trunks. In the office environment it worked well.

On site we could establish calls from both directions, however there was one-way audio when placing a call from the NEC to the Mitel.

After much troubleshooting, it appears to have been the Fortinet Firewall, even though it appeared to have the SIP Helper disabled.

Below is the procedure to deactivate it (kindly provided by a support technician):

--Via CLI interface

config system settings
 set sip-helper disable
 set sip-nat-trace disable
end
--Reboot the fortigate device

--Remove the default-kernel helper entry for SIP

config system session-helper
 show           {//locate the SIP entry, usually 12 or 13, but can vary}
 delete [identified ID] {//the number that you identified from the previous command}
end
--Disable RTP processing as follows:

config voip profile
 edit default
 config sip
 set rtp disable
end--This should disable SIP Helper on most Fortigates, dependant on FortiOS version.