Yes, it's just one of the many ways you could do it, you could create the pages catalogue/phones/ in GS admin and assign a product.php template to phones.
Or you can even use your standard template and just check whether the data is sent, then execute ItemManager's code:
if(!empty($slug) && !empty($_GET['item'])) {
...
Or you can even use your standard template and just check whether the data is sent, then execute ItemManager's code:
if(!empty($slug) && !empty($_GET['item'])) {
...