How can you bind an Indexed property to a control in WPF

Given an instance of the class ThisClassShouldBeTheDataContext as the Datacontext for the view

class ThisClassShouldBeTheDataContext
{
  public Contacts Contacts {get;set;}
}

class Contacts
{
  public IEnumerable<Person> Persons {get;set;}
  public Person this[string Name]
  {
    get 
    {
      var p = from i in Persons where i.Name = Name select i;
      return p.First();
    }    
  }
}

class Person
{
  public string Name {get;set;}
  public string PhoneNumber {get;set;}
}

How can I bind Contact["John"].PhoneNumber to a textbox?

<TextBox Text="{Binding ?????}" />