Refusal to refund, despite evidence!

I canceled my 4G Sim within 2 days of receipt of the sim because the network was so unreliable. However, I was still billed for two months, with no response from the customer service department. They are refusing to refund me, despite proof of my request to cancel under ticket number #***, and the fact I stopped using the sim, assuming it had been canceled as it no longer worked!