Not honouring warranty

I bought a phone as part of a Telkom contract in March last year. At first it worked great, but the battery has been giving an increasing amount of trouble, with the phone losing battery at excessive rates to the point where I now have to almost constantly charge it. The battery has also expanded to the point where the backplate has lifted. When I tried to hand the device in for repair/replacement at Telkom, they refused to honour the warranty saying the backplate has lifted and it's my fault?