I dont know what f/w gave best voip performance, but to get the USB to work the way you want it to, i would upgrade to 1.22b05. If your happy with that, then go ahead and upgrade to 1.31. But in upgrading to get the USB feature, you are going to have to sacrifice (if your current f/w has it) WDS, LAN routing, and 3G USB features, oh, and the inability to downgrade if you upgrade to 1.31.