ZyXEL Communications ISG50-ISDN Application Note page 28

Goal to achieve:
VoIP traffic goes out primarily through WAN1. In case WAN1 is down, it will go out via WAN2.
Condition:
Primary: WAN1; Backup: WAN2
Add WAN trunk for VoIP traffic - Set WAN1 as Active mode, and set WAN2 as Passive mode.
Configuration > Network > Interface > Trunk > User Configuration > Add
28