Revolting Service

I purchased a tablet for my daughter on the 22/1/2016 at Pep Stores in Overport, Durban - MTN being a network. Before end of Feb 2016 the phone was having endless problems and eventually took it in on the 8/3/2016. They requested an invoice and I advised the store manager that unfortunately it was lost, however she managed to trace the sale, journal print and sent the phone to MTN. MTN is totally refusing to fix the phone without an invoice yet a journal print from PEP was given. I have been moved from pillar to post and to-date the phone is still at MTN and not fixed. This is a total inconvenience and I don't understand why misplacing an invoice is an issue here. On the 7/4/2016 MTN sent an sms requesting a payment of R330 to fix a phone that cost R900 and only lasted a month?????? MTN should not sell to public if they can not take liability.