Harassment over account that was paid a year ago

Beginning March 2015 Stone & Stewart made 2 call outs to me, charged R1528 which I paid on 13/3/2015 & sent proof. Every month since then they send a payment reminder and I resend the proof of payment. November 2015 they called to say that they hadn't realised I had paid because I paid one amount & not 2 but they would allocate my payment. The month thereafter I got a payment reminder again etc. Now they have sent a letter saying they have made numerous attempts to recover the money telephonically and are going to hand me over!!! Their telephone lines don't work and they don't respond to their email so you cannot contact them!! I would like them to contact me.