In addition, you might want to consider creating a custom outbound rule for VoIP traffic based on latency results on each WAN.
The following outbound rule defines that all SIP traffic that utilizes UDP port 5060 is forwarded over to WAN1. When WAN1 goes down, WAN2 will take over.
In this case, I suppose that WAN1 has the lowest latency among other WANs.
Simply speaking, it is generally said that low latency becomes important for real-time traffic such as VoIP calls and video streaming.
Having a low latency connection is one of the key factors to deliver a positive user experience for this kind of traffic. There are some other variables that affect VoIP traffic. However, this is something that you might want to experiment.