I think your requirement can be achieved after have better understanding on this.
- WAN1 = 10Mbps
- WAN2 = 100Mbps
Assumed Bandwidth reservation as below:-
- Manager = 50%
- Staff = 25%
- Guest =25%
* Only phone server configured under Manager Group.
* IP for the rest devices put under Staff and Guest Group.
- Enforced phone server (under Manager Group) to WAN1.
- The rest enforced to WAN2
1. Phone server will be guaranteed with 100%(10Mbps) when WAN1 was congested since no others user group using WAN1.
2. Users under Staff and Guest will be guarantee with 50% (50Mbps) each when WAN2 was congested since user in Manager Group not using WAN2. However, if you load balance phone server's traffic into WAN1 and WAN2. WAN1 Bandwidth Reservation remains unchanged, WAN2 Bandwidth Reservation will be changed to Manager 50% (50Mbps), Staff 25% (25Mbps) and Guest 25% (25Mbps).
So WAN2's bandwidth will not wasted. Hope this clear your doubts.