Cell C is billing me to fix my contract phone

I took Cell C phone (Nokia Lumia 925) for 24 months contract that is going to lapse on end March 2016, on the 24 September 2015 my phone was get off failing to start the machine and I return it to the store on the 25 September as is still under warranty, now Cell C they are failing to fix my phone they said I have to make a payment of R2 062.92 for them to fix my phone hence I'm still paying the contract of R447.50 per month, the reference is CLC***. Really this a challenge on my site as I'm still have to pay a contract and the money that they want is unexpected. Please can you assist me to resolve the matter.