Refusing to repair phone under warranty

Last year in June I bought a Blackberry 9720 from a Vodacom store in Westgate Mall using my reward points which I get when I buy their airtime. I gave this phone to my son and about two months ago it stopped charging whenever he put it on a charger. We took the phone back to Vodacom to be checked and repaired since it has a two year warranty. After they checked this phone they said they won't repair it under warranty since it has a physical damage. Their quotation for the repair was above R1450,00. I have a problem with what they said concerning the \physical damage"issue. We are always charging the phone using the charger which came out with the phone. We are charging the phone in the normal on and off process using their charger. Moreover"