Missing payment still not located after almost 4 weeks of paying

It's been 11 days since I wrote to Truworths about a missing payment. I sent their finance department proof of payment and nothing is being done. Is this how u treat ur customers? No one seems to care, all u can about is sending me messages ans calling that I owe u. This is affecting my credit record and I am not happy about it. I am actually angry coz I have been a loyal customer with u guys for years before I paid off my debt and closed my account to recover financially. Now I came back and this is the treatment I get. Sort out my problem and stop blowing up my phone coz I never missed a payment. I paid in time