var intent = new Intent(this, typeof(Activity1));

StartActivity(intent);

var call = new Intent(Intent.ActionCall); //初始化一个电话呼叫

call.SetData(Android.Net.Uri.Parse("tel:13811112222"));

StartActivity(call);

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:orientation="vertical"
 android:layout_width="fill_parent"
 android:layout_height="fill_parent">
 <Button
  android:id="@+id/btn1"
  android:layout_width="fill_parent"
  android:layout_height="wrap_content"
  android:text="打电话" />
 <Button
  android:id="@+id/btn2"
  android:layout_width="fill_parent"
  android:layout_height="wrap_content"
  android:text="转到活动1" />
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:orientation="vertical"
 android:layout_width="fill_parent"
 android:layout_height="fill_parent">
 <TextView
  android:text="这是活动1的界面\n\n提示：按【Back】按钮返回"
  android:textAppearance="?android:attr/textAppearanceLarge"
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:id="@+id/textView1"
  android:layout_gravity="center"
  android:layout_margin="30dp" />
</LinearLayout>

using Android.App;
using Android.Content;
using Android.OS;
using Android.Widget;

namespace MyDemos.SrcDemos
{
 [Activity(Label = "【例12-1】Intent基本用法1")]
 public class ch1201IntentDemoMain : Activity
 {
  protected override void OnCreate(Bundle savedInstanceState)
  {
   base.OnCreate(savedInstanceState);
   SetContentView(Resource.Layout.ch1201_Main);

   //启动电话拨号的Activity
   FindViewById<Button>(Resource.Id.btn1).Click += delegate
   {
    var call = new Intent(Intent.ActionCall);
    call.SetData(Android.Net.Uri.Parse("tel:13811112222"));
    StartActivity(call);
   };

   //启动自定义的Activity
   FindViewById<Button>(Resource.Id.btn2).Click += delegate
   {
    var intent = new Intent(this, typeof(ch1201IntentDemoActivity1));
    StartActivity(intent);
   };
  }
 }
}

using Android.App;
using Android.OS;

namespace MyDemos.SrcDemos
{
 [Activity(Label = "【例12-1】Intent基本用法1")]
 public class ch1201IntentDemoActivity1 : Activity
 {
  protected override void OnCreate(Bundle savedInstanceState)
  {
   base.OnCreate(savedInstanceState);
   SetContentView(Resource.Layout.ch1201_Layout1);
  }
 }
}