With BT-Fon the ADSL owner's traffic has priority over the BT-Fon traffic if push comes to shove.

So someone can use up to 512kbps of my ADSL connection, unless I need it, in which case they get throttled down to nothing.

(The traffic is also segregated in that BT-Fon stuff goes down the wire as a separate IP address from my own ADSL connection data. I'm not sure if multiple people on Fon on a single wireless router get unique IPs or not.)