Incorrect Amount on Invoice

I have a big problem with Telkom. I paid my account via EFT on the 25th of January 2020. I then recieved a bill saying I owe them R365 because when they cane to collect on the 31st of January they didn't find the money. The bill i recieved clearly shows that they recieved the money on the 25th and now im being charged because i paid my account before time!! Which law are they using to stop me from paying my account before its due? I did the sane EFT in February 2020 and they didn't say I owe them money because when they came to deduct on the 28th there was no money. Can someone please sort out my account because my accounts are always paid on time and DON'T WANT TO TARNISH MY CREDIT RECORD BECAUSE OF YOUR INCOMPETENCIES. Thank you in advance.