Example override of field widget

<?php

namespace Drupal\mymodule\Plugin\Field\FieldWidget;

use Drupal\telephone\Plugin\Field\FieldWidget\TelephoneDefaultWidget;

/**
 * Plugin implementation of the 'telephone_input_mask' widget.
 *
 * @FieldWidget(
 *   id = "telephone_input_mask",
 *   label = @Translation("Telephone number with input mask"),
 *   field_types = {
 *     "telephone"
 *   }
 * )
 */
class TelephoneWidget extends TelephoneDefaultWidget {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['value'] = $element + array(
      '#type' => 'tel',
      '#default_value' => isset($items[$delta]->value) ? $items[$delta]->value : NULL,
      '#placeholder' => $this->getSetting('placeholder'),
      ''#attached' => ['library' => 'mymodule/input-mask']
    );
    return $element;
  }

}