Call for Price for WooCommerce Pro Nulled plugin extends WooCommerce by outputting “Call for Price” when price field for product is left empty. You can additionally change the default add to cart button text or hide button completely for products with empty price.
Call for Price for WooCommerce Pro version also allows you to set “call for price” labels on per product basis.
Demo : https://wordpress.org/plugins/woocommerce-call-for-price/