This should have been on Asterisk Support.
Are you saying that your ADSL supplier provides a VoIP service which it terminates on an analogue interface, as distinct from simply splitting out the baseband analogue with a microfilter?
If so, how does the ADSL equipment signal disconnect supervision. As far as I know, Asterisk only supports temporary removal of battery, and an extended period of busy tone, and must be configure accordingly.
There is no mention of tones in the body of the article, but the subject implies you expect Asterisk to produce a tone, but disconnect detection, by tone, requires the exchange side to send the tone.
Generally, if reliable disconnect detection is needed on a circuit switched system, you need ISDN, or an analogue line intended for PABX use.