asterisk users - Dec 2004 - how to make asterisk drop battery on a FXS?

I connected two plain old telephones to FXS lines of a TDM400P (defined as
fxoks in zaptel.conf), one of them dials the other with Dial(Zap/2), I talk
to myself for a while, hang up either of the phones, and the phone that
remains off-hook gets the congestion tone until it goes on-hook (at least as
long as I've cared to wait).  I don't have a voltmeter on the line, but
if
I'm hearing the congestion tone, there's battery, right?

The reason I want to drop battery is that I want to connect some FXS lines
to an old IVR system, and if I dial the IVR and hang up, the IVR doesn't
notice.  Another possibility is to see if I can change either the IVR or
Zaptel (why doesn't the Zap channel get its tones from indicators.conf, like
other channels do, instead of having them compiled in?)  I spent a while
playing with ) so that the IVR recognizes the congestion tone and hangs up,
but battery drop, if I can do it, sounds like a better way to do it.

thanks