BAD MTN REPAIR SERVICE

I bought my 88 year old mother a new Samsung A03 phone at our local pep stores she had the phone for 4weeks after which it did not want to switch on, she took it to Pep store and they sent it away for repairs -- up till today now 6weeks later they cannot explain what the problem is - this phone is under guarantee and in the meantime my mother does not have capable communication. when i bought the phone there was nothing wrong with my money i paid for it so how come the do not reach an acceptable agreement - or pay the money back so that i can look for something else -