Required correct number

Hi, I entered this plugin, I think I activated it correctly. The problem is that if instead of entering a correct mobile number, I also put only the number 5, the email is sent anyway. It is not a correct mobile number.
What should I do?

<div class="contact-form-style-3">

<p class="col-md-4 name">[text* your-name placeholder akismet:author "Nome e Cognome*"]</p>
<p class="col-md-4 email">[email* your-email placeholder akismet:author_email "Email*"]</p>
<p class="col-md-4 phone">[phonetext* phonetext-739 placeholder numberonly "Telefono*"]</p>
<p class="col-md-12 cf-style3-text">[text* your-subject placeholder "Titolo della richiesta*"]</p>
<p class="col-md-12 cf-style3-textarea">[textarea* your-message placeholder "Messaggio*"]</p>
<p class="col-md-12" >[submit "Invia Messaggio"]</p>
</div>

The page I need help with: [log in to see the link]