<?php
function hoccodetot_oft_custom_get_price_html( $price, $product ) {
    if ( !is_admin() && !$product->is_in_stock()) {
       $price = '<span class="amount">' . __( 'Liên hệ', 'woocommerce' ) . '</span>';
    }
    return $price;
}
add_filter( 'woocommerce_get_price_html', 'hoccodetot_oft_custom_get_price_html', 99, 2 );
?>