Faulty contract phone LCD is not working 100%

My wife both a twin deal at vodacom online when they deliver the two phone it was working good but after 2 month that phone A15 Samsung she give me the LCD is faulty if I want to touch the whole screen is jumping and iam working everyday with my phone and I use it for work now iam paying for a contract that is almost R600 a month and how can you pay for a phone that is not working properly when I report it to them they say I already have it and there warranty is only for 7 days what the hell and I have the phone now for 4months they did cell us faulty contract phones