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

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.companyname.DemoPhoneCall">
	<uses-sdk android:minSdkVersion="15" />
	<application android:label="DemoPhoneCall.Android">
	  <provider android:name="android.support.v4.content.FileProvider"
	            android:authorities="com.companyname.DemoPhoneCall.fileprovider"
	            android:exported="false"
	            android:grantUriPermissions="true">
	    <meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/file_paths"></meta-data>
	  </provider>
  </application>
</manifest>