How am i exptected to pay for a contract phone which is not working or factory fault

I booked my contract phone for repairs at Cell C its still fairly new 13 months and still under warranty, but the company has sent me a bill of R3900 for a phone which should be fixed or replaced as the damage to the phone is not my fault the phone started glitching out of the blue. How am i expected to pay for a contract phone that is not working until the end of the contract.