asterisk users - Mar 2005 - determining an available channel question

Hello,

I have an outgoing dial plan which utilizes FWD for any of our
outgoing 800,877,866, etc while, toll and local calls get routed
to one of our pots lines.


when we use the pots lines, we use the chanisavail function
to choose an available pots line.

for example
exten => _1800NXXXXXX,1,ChanIsAvail(Zap/26&Zap/25)
exten => _1800NXXXXXX,2,Cut(theChannel=AVAILCHAN,,1)
exten => _1800NXXXXXX,3,Dial(${theChannel}/w${EXTEN}) 

works like a champ

Recently, we have had to stop using FWD because at times the call
cannot be processed (due to load?? probably). But there is a
voice response indicating the call cannot go through.

Currently, all tollfree calls chew up a pots line.

When this happens, I would like the call to failover to one of
the available pots lines.

ChanIsAvail is supposed to work with SIP from what I read, but
since FWD answers, the call is complete.

Has anybody confronted this type of configuration with any
success? Is it possible? If so, a pointer would be appreciated.

Best Regards
Greg