You could use an uplink Flow preference to push your VoIP traffic to Internet/WAN 2 as an SD-WAN rule.   Look under Security & SD-WAN > Configure > SD-WAN & traffic shaping   Under Flow preferences, Internet traffic (assuming this traffic isn't to be VPN tunnelled?)
 
You would need to be able to define the necessary traffic using these variables:
Protocol; Source IP; Src port; Destination IP; Dst port.   (you can sub Any for specifics, within these)
 
This traffic would fail over to WAN 1, if WAN2 failed.