Generally we don’t recommend having optional fields. If you don’t need information to process an order, asking a customer to review an optional field is unnecessary friction that could hurt your conversion rate.
But if you want to add optional phone fields, this is managed through the WooCommerce customizer settings. Simply click “Customize” at the top of the checkout page (you must be logged in as an admin!) and then click WooCommerce > Checkout.
Set Phone field to ‘Optional’. This enables an optional billing and shipping phone field. We recommend you only enable the phone fields if they are truly neccessary for you to process orders.