What is the best way to set up VoIP on the network?

[shane_onz]

I have a problem with sound disconnecting on VoIP. 

Currently I put everything through Fortigate Firewall including PC, Phone, IP Phone, CCTV. 

I have tried a Loadbalance but it still doesn't work well. 

I am not quite sure whether which is the best setting for Bloabalancing. 

Is there a way that I could set up Fortigate so that the phone system won't slow down the internal network usage?

Should I just use two separate ISP for internet and VoIP?

Here is my Physical setup

Interface 1 (internal)

Interface 3 (ISP1)

Interface 4 (ISP2)

Many thanks :D