Hi Gary,
It is not a good idea to call external web service from workflow for many reasons. One given here:
1) If web service is not available, it does not notify user for any failures.
Can you write the same code in plug-in and check if this works for you? If it does and you still need to have workflow, suggest check the async account running the service and make sure it has enough rights to call web service
and also make sure async box has internet connection to make the outbound service call.
Hope this helps.
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Minal Dahiya
blog : http://minaldahiya.blogspot.com.au/
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