How to dial a Number in C# windows universal 10
As your error states:
The type name
PhoneCallManagercould not be found in the namespaceWindows.ApplicationModel.Calls. This type has been forwarded to assemblyWindows.Foundation.UniversalApiContract, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null, ContentType=WindowsRuntime. Consider adding a reference to that assembly.
So you need to add a reference (see MSDN: How to: Add or Remove References By Using the Add Reference Dialog Box) to the assembly Windows.Foundation.UniversalApiContract.
If you search the web for the "consider adding a reference...", you will find you need to reference the Windows 10 SDK, not the Windows Phone 8.1 SDK.
The reference you need to add for this assembly is under "Universal Windows" -> "Extensions" and is called "Windows Mobile Extensions for the UWP".