The web application at http://xxx.xx.xx.xx could not be found
I was receiving the following error when trying to call a web service externally using the IP address.
The web application at http://xxx.xx.xx.xx could not be found. Verify that you have typed the URL correctly. If the URL should be serving existing content, the system administrator may need to add a new request URL mapping to the intended application.
Adding alternate access mappings for web application in Central Administration sorted out the issue.