Example of how make a phone call in Xamarin.Forms applications - http://julianocustodio.com/chamada-telefonica-xamarin-forms

MainPage.xaml.cs

using System;
using Plugin.Messaging;
using Xamarin.Forms;

namespace DemoPhoneCall
{
	public partial class MainPage : ContentPage
	{
            public MainPage()
	    {
		InitializeComponent();
            }

	    public void Call(object sender, EventArgs e)
	    {
	        var phoneDialer = CrossMessaging.Current.PhoneDialer;
	        if (phoneDialer.CanMakePhoneCall && !String.IsNullOrWhiteSpace(Number.Text))
	            phoneDialer.MakePhoneCall(Number.Text);
            }
        }
}