We had an office that was using Vontage for VOIP services. When we rolled in a new FortiWifi 90D in, they started to have issues. Two out of three times, the inbound call had no audio but the third time, it would work. To resolve this issue, we did the following.
Enter the following commands from CLI:
config system settings
set sip-helper disable
set sip-nat-trace disable
You will then want to reboot the device.
One the box has rebooted, open the CLI and enter the following commands – do not enter the text after //:
config system session-helper
show //locate the SIP entry, usually 12, but can vary.
delete 12 //or the number that you identified from the previous command.
Disable RTP processing as follows:
config voip profile
edit default
config sip
set rtp disable
Colin Weiner - East Side - West Side 