Your problem is probably not bandwidth (Score 1)

It's most probably latency, the router doesn't have enough juice to push voice packets through at the speed you want it to, especially when there is some "legal" bittorrent going on - remember, bittorrent takes a lot of your upload speed, not just download, uploading what might "be legal" or not, putting you in the p2p network. Again, how much of queuing would these home routers do?