refusing to exchange/ refund adamaged cellphone

i bought a phone at woolworths in jabulani mall ( a samsung j1, vodacom) in august before even 30 days passed by i had a problem with the speaker so i took back the phone to woolies, they returned the phone and after few days the problem resumed again , i took the phone back again and the same thing happned.. the problem was when i took it for the third time, they refused to take it for repairs and told me vodacom said they cant find any fault with the phone even though i told them the speaker has a problem.. they dont want to exchange, refund or repair the phone anymore. please help, im really dissappinted at both woolworths and vodacom