The code is located in iOS/Android project space , how to access these code from the PCL project!

In my Xamarin Studio for MAC I use "Xamarin.Forms" solution. I've 3 projects. PCL , iOS and Android.
I've a proxyclass (webservice class) which I copied the file "checkServices.cs" into the iOS and Android projects. I also added the System.Web.Webservices reference in the "iOS" and "Android" project.

The PCL is referenced in iOS and in Android, but can not reference vice versa. The sharing is only single way. from PCL --> iOS/Andoroid!

Now how can I call these methods from my PCL to put the data on XAML page ? I like to call the methods from PCL which are located in iOS/Android project folder.