/**
* Changes price to "Call for Price" for all products with empty price.
*
* @see https://wpcodebook.com/snippets/display-call-for-price-for-products-in-woocommerce/
*/
add_filter( 'woocommerce_empty_price_html', function ( $price ) {
return __( 'Call for Price' );
}, PHP_INT_MAX );