Can we assume you are in the UK?
That is the busy tone not generally the same as the BT "Howler tone"
so disconnect supervision needs to match what the provider sends polarityswitch or regular CPC
http://www.sandman.com/cpcbull.html
if neither then busydetect will just notice repetitive tones on the line, I would stick a butt set on the line and try and diagnose if the telco doesn't provide disconnect supervision and make your best guess
