title: Custom Filter in Django admin
body: <p>I have a model as follows</p>
  
  <pre><code>class Person:
    name = models.CharField()
    city = models.CharField()
    phone = models.CharField()
  </code></pre>
  
  <p>I want to create a filter in the admin page, the filter should be based on phone, that is </p>
  
  <pre><code>valid phone(having 10 digits)
  invalid phone
  </code></pre>
  
  <p>I don't want to create a validation. I just want to filter out who has a valid phone and invalid. 
  Thanks </p>