Nuisance calls

I am a pensioner with cancer and I keep getting harassed by Tfg calling me to pay my account as if I have ever skipped a payment. I keep paying and on time no matter what yet they keep calling me telling me to pay more than what my grant is a month and they threatened to list me. Seriously I have been a valued client for so many years and never falter in my payments. I made an arrangement with head office to pay R450 but no I keep getting calls to pay r1500 my whole grant. Really .I know to pay my accounts so stop calling me as I can only afford the r450. Not more