If you want to make a phone call from a Windows Phone 8.1 App (XAML App) , you can use the ShowPhoneCallUI method that is defined in the Windows.ApplicationModel.Calls.PhoneCallManager class.

How to make a phone call in Windows Phone 8.1 App ?

Windows.ApplicationModel.Calls.PhoneCallManager.ShowPhoneCallUI("<Phone Number>", "<Display Name>");