Useless service

<p>so I returned a router that was faulty and they gave us a gift card which then we managed to missplace. The store phoned and told us that they still have the slip and we should come in store, when I got there some rude manager interrupted the lady at the counter helping me and said we would have to phone their customer service and that they can't help us. I've now spoken to some one at each of the following numbers ********** 120 , ********** 775, and ********** 700 all putting me on hold for 10 min and then just referring me to the next person. I have a photo of the slip with the voucher number and it has been confirmed that it hasn't been used what do I need to do to get either my money back or the voucher reprinted?</p>