Poor customer service

I purchased a Hisense phone for my dads 70th birthday less than 3 months ago... then on the 15th June the phone switched off and he immediately told me and I said I will take it in to the store the very next day. The next day I take it in and they tell me they going to book it to have it assessed, already on that she tells me they might charge me R300 for assessment, I was like "since when, the phone is still new and clearly its under warranty still" In any case she takes it and books i***** been 2 weeks since then and heard nothing so I went into the store to quarry, they cashier (Michelene) called to do a follow up, and they state that they cannot fix the phone because the phone was tampered with "someone tried to open it" By now I am fuming because I know for a fact that is a lie, no1 touched the inside of the phone only them, I continue to ask for proof and she says they can only give it to me on Monday, from now until Monday anything can happen create tampering... I feel that this is a money making scheme and I want my money back or another phone or I am taking this further to the public protector. I mean less than 3 months and they want me to pay for a broken phone my word this is not on... Also upon booking the phone we asked does this phone have the issue of switching off, and the cashier who booked the phone said she does not know, but in her records there was a man who booked his phone for the same problem and he was also told to b pay a fee to fix his phone... Day light *******, this matter better be sorted out soon.