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

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:DemoPhoneCall"
             x:Class="DemoPhoneCall.MainPage">

    <StackLayout VerticalOptions="CenterAndExpand">
        <Entry x:Name="Number"  Keyboard="Telephone"/>
        <Button Text="Call" Clicked="Call"/>
    </StackLayout>

</ContentPage>