Web Service "Request failed with status: Unauthorized."

I have created a custom web service on a sharepoint 2010 server and am able to access it in a webpage on my server like so:

http://localhost/_vti_bin/TestWebService.asmx

Although when I try to access it in the Call web service action like this:

Web URL/_vti_bin/TestWebService.asmx

I get the "Request failed with status: Unauthorized." error. Any suggestions?