Chapter 22 VoIP Solutions: SIP : FortiGate VoIP solutions: SIP : The SIP ALG : SIP ALG configuration overview : Disabling the SIP ALG in a VoIP profile
  
Disabling the SIP ALG in a VoIP profile
SIP is enabled by default in a VoIP profile. If you are just using the VoIP profile for SCCP you can use the following command to disable SIP in the VoIP profile.
config voip profile
edit VoIP_Pro_2
config sip
set status disable
end
end