asterisk users - May 2006 - zt hook failed

Asterisk 1.2.7.1, Red Hat 9.0, TDM 400 2 FXS, 1 FXO

I'm getting the following warning from time to time:

     "zt hook failed: Device or resource busy"

It seems that once I get this error I can no longer use my zap
channel.  Interestingly it seems to affect SIP as well as I can no
longer dial out on that either, it begins to complain that my GSM
licenses are used up (I only have one).

Any ideas how I defeat this nasty, intermittent problem?

Thanks,
H