/* Show contact form instead of "Out Of Stock" message */
add_action('woocommerce_single_product_summary', 'out_of_stock_show_form', 20);
function out_of_stock_show_form() {
    global $product;
    if(!$product->is_in_stock( )) {
        echo  '<div class="oos-form">';
        echo  '<b>Bu Ürünün Stok Bilgisi Hakkında Bilgi Almak İstiyor musunuz ?</b>:<br>İletişim :';
        echo  do_shortcode('[whatsappsupport number="[COLOR=#000000][FONT=Open Sans]Telnumarası[/FONT][/COLOR] " text="Whatsapp Destek" text-color="#fff" bg-color="#22c15e" message="Merhaba" ] [maxbutton id="1"]');
        echo  '</div>';
    }
}
 
/* This one will work on Variable Product - When a Variation is out of stock */
add_filter( 'woocommerce_available_variation', 'variation_out_of_stock_show_form', 10, 3 );
function variation_out_of_stock_show_form( $data, $product, $variation ) {
    if( ! $data['is_in_stock'] )
    {
        $data['availability_html'] = '<div class="oos-form">';
        $data['availability_html'] .= '<b>Bu Ürünün Stok Bilgisi Hakkında Bilgi Almak İstiyor musunuz ?</b>';
        $data['availability_html'] .= do_shortcode('[whatsappsupport number="Telnumarası" text="Whatsapp Destek" text-color="#fff" bg-color="#22c15e" message="Merhaba" ]');
        $data['availability_html'] .= '</div>';
    }
    return $data;
}