ITC and default judgment threats

My doctors account of about R300 was handed over to Nicol Davies etc for collection. I then paid the account on 22 January 2015 and emailed them proof of payment (more than once). I got a call from a very rude woman who wanted payment and when I told her that I had paid and that she must check her records, she threatened to list me with the ITC. I now get smses daily from Nicol Davies etc saying \You have been in default of your account due to Nicol