Require Phone Number for WooCommerce Subscriptions

<?php
/**
 * Plugin Name: Require Phone Number for Subscriptions
 * Plugin URI: https://codewp.ai
 * Description: Makes the Phone Number field required only for subscription products.
 * Version: 1.0
 * Author: CodeWP Assistant
 * Author URI: https://codewp.ai
 */

/**
 * Checks if the product in the cart is a subscription and makes phone field required.
 */
function cwpai_require_phone_for_subscriptions() {
    if ( class_exists( 'WC_Subscriptions_Product' ) ) {
        foreach ( WC()->cart->get_cart() as $cart_item ) {
            $product = $cart_item['data'];
            if ( WC_Subscriptions_Product::is_subscription( $product ) ) {
                add_filter( 'woocommerce_billing_fields', 'cwpai_make_phone_field_required' );
                break;
            }
        }
    }
}
add_action( 'woocommerce_before_checkout_form', 'cwpai_require_phone_for_subscriptions' );

/**
 * Makes the phone field required.
 * @param array $fields Billing fields.
 * @return array Modified billing fields.
 */
function cwpai_make_phone_field_required( $fields ) {
    $fields['billing_phone']['required'] = true;
    return $fields;
}