Does Dual WAN Work? With QoS?

[Authority]

Hi I've asked this before and I don't think I ever got an answer. I am thinking of adding an additional WAN line but it's kind of pointless of the dual WAN feature doesn't work that well (or at all). I remember reading at one point it was broken from Asus?

I am using the line for 20 VoIP phone too, and not sure how that will work with QoS (if at all)?

Current WAN: 100/20 Sonic.net and thinking of adding a second line of same speed.

Thanks.