DAHDI reports an outgoing call answered on faulty line

My Asterisk system was working fine until a few days ago when a line fault occurred. I have a TDM 410 card connected to two analogue lines. When making an outgoing call through the PBX, Asterisk CLI reports:

Called DAHDI/g0/<called number> DAHDI/1-1 answered SIP/214-00000002 Hanging up DAHDI/1-1

There was a line fault and no dial tone on the first line Dahdi tried, yet Dahdi reported it answered. The other line was available and working, but never got tried by Dahdi. Does anyone know how to get a Dahdi channel to report “un-available” or “line-fault” rather than “answered” if there is no dial tone?