asterisk users - Sep 2005 - How to suppress Local/Zombie channels?

hi,

can anyone please tell me under which circumstances asterisk creates
Local/Zombie channels and how to suppress this? It only seems to happen when a
user calls himself, but I can't reproduce this in our testsystem and it only
happens occasionally. All we do in the extensions.conf is send the incoming sip
call control out to a FastAGI server, which does the actual Dial command. We
don't consciously create any Local channels, its definitely asterisk who
decides that theres a need for it ...

Any help appreciated,
thanks a lot,
Tony