title: use "grep" command to find txt files containing telephone number
body: <p>use "grep" command to find txt files containing telephone number. </p>
  
  <p>telephone number format could be:</p>
  
  <pre><code>"***-*******"
  "**********"
  "*** *******"
  "***-***-****"
  </code></pre>
  
  <p>How to write it?</p>