You should look at chan_dahdi.conf.sample, starting at:
;
; On trunk interfaces (FXS) and E&M interfaces (E&M, Wink, Feature Group D
; etc, it can be useful to perform busy detection either in an effort to
; detect hangup or for detecting busies. This enables listening for
; the beep-beep busy pattern.
;
;busydetect=yes
and at:
; For FXS (FXO signalled) ports
; switch the line polarity to signal the connected PBX that the current
; call was “hung up” by the remote party
; For FXO (FXS signalled) ports
; In some countries, a polarity reversal is used to signal the disconnect of a
; phone line. If the hanguponpolarityswitch option is selected, the call will
; be considered “hung up” on a polarity reversal.
;
;hanguponpolarityswitch=yes
I thought there was an option to use a temporary open circuit, as well, but I can’t find it.