Understanding SIP ALG Security

We have deployed the new sip gateway behind our firewall(FortiGate).As per suggested by Fortinet I have enabled the VoIP profile on firewall policy to protect the network from SIP attacks.

I just wanted to understand how SIP ALG protects the network from SIP attacks?

I have gone through the FortiGate cookbook for the SIP ALG, but I did not find satisfactory answers. Some videos suggest to turn off the SIP ALG as it modifies the sip packets in unexpected ways resulting correct the packets and making them unreadable. This cause the unexpected call drop or unreachable.

Can someone please help me to understand what should I do to secure SIP traffic.