1. you need oen the reverse polarity signal to detect otherside answer of hangup.
after you open reverse polarity signal from the provider. edit the /etc/asterisk/chan_dahdi.conf
and command the busydetect and busycount parameter.
1. if you have reverse polarity signal for answer and hangup, you should command busydetect and busycount parameter.
2. you mean same pstn line before work well on same card but recently can't detect remote answer signal?
3. you can add my skype:dario.hu2 and prepare teamvidw for check.