Hello,
Why would you want to call a web script from a workflow listener? Are you trying to access another Alfresco instance from the one that executes the workflow? Otherwise you would not need to do so, since you are already within an Alfresco repository and can access any necessary services via JavaScript or Java listeners (via the script APIs or foundation services).
Some more information on what you are trying to accomplish may be useful here…
Regards
Axel