If you want to courier a phone, please remove the battery when sending. Phone batteries are classified as DG (Dangerous Goods) and are not allowed to be carried by most couriers.
For repair and return items, it should be declared as C2 or K2 item, do tell the courier service that this is a c2 item, to avoid any duty charges on either end. Just remember to prepare a commercial invoice and state that it is a R&R item. I would suggest DHL or FedEx only. The rest are not so reliable.