Grid.php переопределение не работает

Я пытаюсь override ниже файла:

/app/code/core/Mage/Adminhtml/Block/Newsletter/Subscriber/Grid.php :

class Mage_Adminhtml_Block_Newsletter_Subscriber_Grid extends Mage_Adminhtml_Block_Widget_Grid
{
   ...
   protected function _prepareColumns()
   {

      $this->addColumn('subscriber_id', array(
         'header'    => Mage::helper('newsletter')->__('ID'),
         'index'     => 'subscriber_id'
      ));

      /* Need to add own cloumn */

   }
   ....
}

Вот что я пытаюсь сделать с custom module: /app/code/local/Customletter/Phone/etc/config.xml:

<?xml version="1.0"?>
<config>
        <modules>
            <Customletter_Phone>
                <version>1.0.0</version>
            </Customletter_Phone>
        </modules>

    <global>

       <blocks>

             <phone>
                <class>Customletter_Phone_Block</class>
             </phone>

             <adminhtml>
               <rewrite>
                     <subscriber_grid>
                         Customletter_Phone_Adminhtml_Block_Newsletter_Subscriber_Grid
                     </subscriber_grid>
               </rewrite>
             </adminhtml>

      </blocks>


     <resources>
        <customletter_phone>
            <setup>
                <module>Customletter_Phone</module>
            </setup>
            <connection>
                <use>core_setup</use>
            </connection>
        </customletter_phone>
    </resources>

    </global>  

</config>

/app/code/local/Customletter/Phone/Adminhtml/Block/Newsletter/Subscriber/Grid.php:

class Customletter_Phone_Adminhtml_Block_Newsletter_Subscriber_Grid extends Mage_Adminhtml_Block_Newsletter_Subscriber_Grid
{

   protected function _prepareColumns()
   {


     $this->addColumn('customletter_phone', array(
        'header'    => Mage::helper('newsletter')->__('Customer Phone Number'),
        'index'     => 'customletter_phone',
        'default'   =>    '----'
    ));


        return parent::_prepareColumns();
  }
}

Проблема: В Newsletter Subscribers grid Customer Phone Number не появляется.

Я использую magento 1.9.2.4.