title: Linq query to filter contacts having same phone number
body: <p>I have a list of contacts where each contact (saved in Contact table) can have multiple numbers (saved in another table Contact_Phones)</p>
  <pre><code>public class Contact{
      public int ID {get; set;}
      public string First_Name {get; set;}
      public string Last_Name {get; set;}
      public List&lt;Contact_Phones&gt; Phones {get; set;}
  }
  
  public class Contact_Phones{
      public int ID {get; set;}
      public int Type {get; set;}
      public string Phone_No {get; set;}
  }
  </code></pre>
  <p>Now I want to filter contacts having same phone_no using Linq C# and merge those contacts.</p>