asterisk users - Jun 2008 - how to have an agi check for dial tone on analog lines before dialing

hi,

I have an AGI running after an outgoing call file starts it up.
Everything works fine except if my line has a problem.
Trying to simulate this I unplug the line. So there is no dialtone.
How do I detect this and let the AGI know so I can try line 2, 3, 4 etc...

Detecting the the AGI or some other way is fine. I just need to know.
I am using a TDM804B card at this time.

Jerry