Read original post at Mohamed Yamama's Blog
I’ve got lots of queries recently from fellow developers about their inability to consume WCF services from Windows Phone 8.1 Runtime app. Apparently, adding a service reference (for WCF Services) to the project via Visual Studio is currently available for Windows Phone 8.0, Windows Phone 8.1 Silverlight and Windows 8.1 projects but not for Windows Phone 8.1 Runtime project.