if get_post_meta do something

If you mean you don’t want the <div class="phone-num"> to appear at all, here’s how:

<?php
$phone_number = get_post_meta(get_the_ID(), '_custom_product_number_field', true );
if ( ! empty( $phone_number ) ) : ?>
  <div class="phone-num">
    Phone: <?php echo $phone_number; ?>
  </div>
<?php endif; ?>