Yojance on "[Plugin: WooCommerce - excelling eCommerce] Make phone number not require"

The very last piece of code on that page is what you need to insert into your functions.php file inside your WordPress theme.

add_filter( 'woocommerce_billing_fields', 'wc_npr_filter_phone', 10, 1 );

function wc_npr_filter_phone( $address_fields ) {
	$address_fields['billing_phone']['required'] = false;
	return $address_fields;
}