UNKNOWN CHARGES

My contract recently came to and end with Cell C. When I spoke to the sales person about an upgrade, I agreed to a top up contract at R140 per month. But I'm being charged R500 or sometimes R800 on my account still. When I phoned in to ask why it's still a high bill they said its because my handheld is still being paid off. But this has gone on for too long now and my device is definitely paid off by now.