Change tax class if field in checkout is not empty in WooCommerce

Question

I am using standard tax rate in WooCommerce but if I want to sell to business customer in other country, then VAT rate must be lower. How can I change tax to "Reduced rate" if field with ID "billing_ic" is filled?

I am tried this, but it does not work:

function wc_different_rate( $tax_class, $product ) {
    $checkout = WC()->checkout();
    $billing_ic = $checkout->get_value( 'billing_ic' );

    if ($billing_ic == '') {
        $tax_class = 'standard-rate';
    }
    else {
        $tax_class = 'reduced-rate';
    }
    return $tax_class;
}