asterisk users - Oct 2004 - Detecting Busy when dialing out on ZAP channel.

Hi list,
as the subject says, is it possible to detect that the number you call 
on a pstn channel is busy - and then to do somthing else. I've tried the 
{contect}+101 with the dial command but that doesnt kick in. I guess 
thats there for 'there are no zap channels to dial out with' rather than
the number you dialed is busy.

Thanks
Wayne.