REFUSE TO HONOUR WARRANTY

I have purchased a Sony Xperia during June 2013 from MTN.Approximately one month ago I discovered that my phone does not charge.When I disconnect the charger from the phone it will not turn on.I then decided to send it off to MTN repair centre at the end of January 2015.Last month I received an sms advising that I have to pay R1 354.13 for the repairs to be done. At first I was confused, since my cell phone carries warranty of 24 months.When I refused to accept the quotation (and ultimately pay for the repairs) the phone was returned to me.I called the MTN repairs yesterday and spoke to Lerato Sefadi (***),to establish as to why I have to fork out money to pay for repairs even though my phone is under guarantee.She confirmed that the charging socket/USB connector is damaged and the defect it is not a manufacturing fault.It is so easy to imply that negligent handling by the customer of the product is the cause of the damage.Why do we as the consumers have to honour our monthly payment commitment towards our cellphone contract,but MTN refuses to honour the warranty my cellphone still carries up until June 2015.I have pay for repairs & contract for a phone that is not in use