The validation fails as it should but does not return the error message.

       $form->addElement('text', 'phone_number', array(
     'required' => true,
       'validators' => array(
         array('NotEmpty', true, array('messages' => 'Enter a valid Phone Number')),
           array('regex', false, array('pattern' => '/\([0-9]{3}\)\s[0-9]{3}-[0-9]{4}/',
              'messages' => 'Enter a valid Phone Number'
     )),
           'CheckPhoneNumber'),

       ),
   ));

Custom Class:

class Custom_Validators_CheckPhoneNumber extends Zend_Validate_Abstract{
const IN_USE = 'inUse';

protected $_messageTemplates = array(
    self::IN_USE => "'%value%' is currently in use"
);

public function isValid($value)
{
    $this->_setValue($value);

        $user_check = Users::getActive(preg_replace("/[^0-9]/", "", $value));
        if($user_check->id){
            $this->_error(self::IN_USE);
            return false;
        }

  return true;
}

}

Just fails does not give the "IN_USE" error.