admin.order.edit.index.initialize:
 - [onFormAdminOrderEditInit, NORMAL]

public function onFormAdminOrderEditInit(EventArgs $event)
{
    $builder = $event->getArgument('builder');
    $builder->add('tel', 'tel', array(
        'required' => false
    ));
}

            ->add('tel', 'tel', array(
                'required' => false,
                'options' => array(
                    'constraints' => array(
                        new Assert\NotBlank(),
                    ),
                ),
            ))

            ->add('tel', 'tel', array(
                'required' => false,
                'options' => array(
                    'constraints' => array(
                        // new Assert\NotBlank(),
                    ),
                ),
            ))