drupal7_extra_display_fields.php

<?php

/**
 * Two hooks are necessary for adding an extra field to an entity display
 */
/**
 * Implements hook_entity_view().
 * Define the content that should be shown
 */
function hook_entity_view($entity, $type, $view_mode, $langcode) {
  if ($entity->type == 'event') {
    $gateway = sms_gateways('gateway', 'twilio');
    $number = $gateway['configuration']['sms_twilio_number'];
    $field = [
      '#title' => t('SMS Phone Number'),
      '#type' => 'item',
      '#id' => 'sms-number',
      '#markup' => $number,
    ];

    $entity->content['sms_number'] = $field;
  }
}

/**
 * Implements hook_field_extra_fields().
 * Register the field so it can be managed in "Manage Display"
 */
function hook_field_extra_fields() {
  // Often you'll want to loop over entity_get_info()
  // unless you know exactly what entity_type and bundle you're targeting
  // This array is $extra[ENTITY_TYPE][BUNDLE_TYPE][VIEW_MODE][FIELD]
  $extra['event']['event']['display']['sms_number'] = [
    'label' => t('SMS Phone Number'),
    'description' => t('The number to text SMS Event Code to'),
    'weight' => 0,
  ];

  return $extra;
}