Add extra fields to Billing Form

addFieldBillingForm.php

<?php

/**
 * @snippet       Añadir nuevo campo al billing form
 */
  
add_filter('woocommerce_billing_fields', 'custom_woocommerce_billing_fields');

// Backend: Display editable custom billing fields
//add_filter( 'woocommerce_admin_billing_fields' , 'add_admin_custom_fields' );

function custom_woocommerce_billing_fields($fields)
{

    $fields['billing_phone2'] = array(
        'label' => __('Teléfono 2', 'woocommerce'), // Add custom field label
        'placeholder' => _x('', 'placeholder', 'woocommerce'), // Add custom field placeholder
        'required' => true,
        'clear' => false, // add clear or not
        'type' => 'text', // add field type
        'class' => array('input-text'),    // add class name
        'description' => 'Teléfono alternativo',
        );    
    
    $fields['billing_cif'] = array(
        'label' => __('CIF/NIF', 'woocommerce'), // Add custom field label
        'placeholder' => _x('', 'placeholder', 'woocommerce'), // Add custom field placeholder
        'required' => true,
        'clear' => false, // add clear or not
        'type' => 'text', // add field type
        'class' => array('input-text'),    // add class name
        'description' => 'Codigo Identificacion Fiscal',
        );


    return $fields;
}