General Your VoIP provider should give you the information, if the SIP ALG on the Fortigate is needed or not. In the default setting of a Fortigate the SIP ALG is active.
Disable the SIP ALG/Session Helper on the Fortigate

General Your VoIP provider should give you the information, if the SIP ALG on the Fortigate is needed or not. In the default setting of a Fortigate the SIP ALG is active.