title: How do I merge duplicate rows into one on a DataFrame when they have different values
body: <p>I have a <code>DataFrame</code> like the following:</p>
  
  <pre><code>ID  NAME    TEL_1   TEL_2   TEL_3
  1   John    123456  754987  465317
  1   John    465987          465987
  1   John            546783
  2   Robert  264687  
  2   Robert          462531  
  3   William 432645  765346  875137
  </code></pre>
  
  <p>I need to merge the rows that have the same <code>ID</code>, saving the phone values, like this:</p>
  
  <pre><code>ID  NAME    TEL_1   TEL_2   TEL_3   TEL_4   TEL_5   TEL_6
  1   John    123456  754987  465317  465987  465987  546783
  2   Robert  264687  462531  
  3   William 432645  765346  875137  
  </code></pre>