VOIP/SIP policy route?

That part has probably nothing to do with either SIP ALG or SIP session helper, which works at L4 or above. The question is how you set up the failover. Likely you don't have two default routes on your routing table (get router info routing-table all). You need to have that to be able to make the policy route work. I recommend set a higher priority number on the WAN2 default route, like 10. Then all other traffic initiated from inside of FGT would take WAN1 while policy route takes the specific traffic to WAN2.

Or if your phones GW is on the internet, you can just set a specific static route toward WAN2.

Just be aware, in either way, when WAN2 goes down, phones die. If you want to fail it over to WAN1, you need to know the GW IP(s) and do the same way for the default route but swapping the role of wan1 and wan2, then let the second link-monitor take care of it.