Get ID from database when a specific td is clicked

Since your rows are displayed by a foreach loop, you can add the row's ID to the url right away:

PHP Code:

 echo "<td><a href='http://managementtool/index.php/Home/terugbel/" . $row->id . "'>" . $row->terugbellen . "</a></td>"; 


Personally, I prefer to avoid echoing out the complete html and php code, because it's difficult to read (and maintain), due to the numerous double and single quotes.
In your view, you can do this to make it more readable:

PHP Code:

<?php foreach($query->result() as $row) : ?>

<tr>
   <td><?= $row->klantnaam;?></td>
   <td><a href="http://managementtool/index.php/Home/terugbel/<?= $row->id;?>"><?= $row->terugbellen;?></a></td>
</tr>

<?php endforeach; ?>

<?= is shorthand for <?php echo