function savvy_not_required_fields( $f ) {
unset( $f['billing']['billing_last_name']['required'] ); // that's it
unset( $f['billing']['billing_phone']['required'] );
// the same way you can make any field required, example:
// $f['billing']['billing_company']['required'] = true;
return $f;
}
add_filter( 'woocommerce_checkout_fields' , 'savvy_not_required_fields', 9999 );