Badgering customers and inability to listen

For the month of Feb 2015 i had paid short on my instalment due to an unforseen incident. I was well aware that i would need to pay the extra amount at the end of March. on the 23rd of march a call center agent called me for the overdue amount, which i specified that i only get paid on the 25th and will make payment shortly after.(looking at history i only pay after the 25th so common sense and reading the screen should be beneficial however i can understand that this may be too hard a for truworths collections agent). she asked me about how i would make payment and i said the exact same way i do so EVERY month, via EFT.on the 26th at about 12 midday i made the payment.however around around 2pm i started being hounded almost every 2 hours by agents.(inability to understand that payment was made so wait for it to clear) this happened throughout that saturday, then again the next week. Yesterday i received a letter of demand (failure to keep my promise apparently) dated 30/03/2015. Now i cant understand the logic here. I promised after the 25th and i did so on 26th. Why is there an option to pay via EFT if it takes long and we are still hounded by rude, annoying, stupid agents....