When you get an error from an API call, it can be tricky to tell if the error is coming from the API Manager (WSO2) or from the web service itself. (Sometimes we call that the backend web service since it is hidden behind the API Manager gateway.) Figuring out where the problem lies can help you talk to the right person faster.
Brayden has written up a helpful document describing ways you can tell the difference.
There is also a handy table that shows what the various WSO2 API Manager error codes mean.