Refuse to fix phone under Guarantee

On 02/02/2016 my BB9720 did not want to charge with the BB charger nor with any other slim charger. I said to my husband that I would like to go the the Vodacom store at Centurion mall. He said to me is it wise as he only gets bad service. I said yes as I always get good service. What a bad experience I'm experiencing. Handed in my phone and I was told that it can take up to 21 days to fix my phone (bought 18/12/2014 which means my phone is STILL UNDER GUARANTEE). 3 days later I got a call saying that I have to pay for the repairs as I have tampered with the mother board/charger connection. So Didi from Head Office contacted me as I refused to pay for the repairs. She then contacted me advising there is manufacture default and the manufacturer took the phone out of the Guarantee. What utter *******. I'm not a gadget person and I'm not very good with computers either. I can only write e-mails, what's app, sms and google. I don't even know how to create an Excell spread sheet! She offered 3 cell phones to me BB9320 for free, 9720 @ R1300-00 which we pay half/half and a 9790 at R1800-00!! Why do I have to pay for a manufacture default on a phone that I paid R1800-00 for?