autofil_users_info_at_checkout.php

<?php
/*
Plugin Name: Autofill users info at checkout
*/


function autofill_users_info_at_checkout_update_mata( $user_id ) {

    add_user_meta($user_id, 'billing_phone', '0123456789');

}
add_action( 'user_register', 'autofill_users_info_at_checkout_update_mata', 10, 1 );

autofil_users_info_at_checkout_old.php

<?php
/*
Plugin Name: Autofill users info at checkout
*/


function autofill_users_info_at_checkout_update_mata( $user_id ) {
    // First attempt
    update_user_meta($user_id, 'billing_phone', $_POST['user_login']);
    
    // Second attempt
    update_user_meta($user_id, 'billing_phone', get_user_meta( $user_id, 'user_login', true));


}
add_action( 'user_register', 'autofill_users_info_at_checkout_update_mata', 10, 1 );