In this case you would just need to connect both ISPs to the Balance 20. Then create either NAT Mappings or portforwards to tie the Public IP’s to the respective LAN hosts. Once done, you can create QoS rules to prioritize traffic. Also, if you want VOIP to go over a specific ISP and the other as a secondary to VOIP you can create outbound policy rules which govern how traffic is routed out through the Balance. Balance 20 is not able to be deployed in Drop-In Mode.
A easy way to do this would be to enable VLANs (click the (?) next to the LAN interface IP of B20) on the Balance 20 and then create another network for VOIP. Then in the outbound policy rule section you would add a rule defining the VOIP network and choose Priority Algorithm which will allow you to specify what WAN in sequential order it goes out on. QoS will help if you have other data going across that WAN by prioritizing VOIP over data if the WAN is being used by both.