optional-phone-fields.php

<?php
// Do NOT include the opening php tag.
// Place in your theme's functions.php file

// Make phone fields optional
add_filter('woocommerce_checkout_fields', 'unrequire_phone_fields' );

function unrequire_phone_fields( $fields ) {
	$fields['billing']['billing_phone']['required'] = false;
	$fields['shipping']['shipping_phone']['required'] = false;

	return $fields;
}