asterisk users - May 2005 - SIP forwarding to ZAP and call files

I am having trouble with call files and the call forwarding feature.

When I create the call files they are as follows:
Channel: SIP/600
Callerid: My ID
MaxRetires: 3
RetryTime: 150
WaitTime: 45
Context: my-custom-app
Extension: 600
Priority: 1


And when I setup call forwarding I use ZAP channel 1-1 which is
extension 200. So, 600 forwards to 200. It works when dialing into the
system, but fails when trying to make the same thing happen with call
files.

If I setup the call file to dial the ZAP/1-1 extension it works fine,
but I want to be able to setup a SIP to forward to a ZAP and it still
work.

I cannot figure out why this won't work.