Who's going to pay the interest?

I requested my banking details to be updated for my home loan account, as I initially had a stop order but I had switched banks. I was told to write a letter to that effect, which I did on the 24th Dec and I received a confirmation sms that my details have been updated. Today, I discover that no money was debited (Debit order amount was R0) from my account but I also discover from Andile Mngomezulu that on the letter I needed to include that I need to activate the Debit Order on my personal account. Why would I tell the them to change my banking details if I didn't want them to take the money from my account for now until I request a stop order from my bank? Why was I also told that the Debit order that was going to go off my account with the actual installment instead of with the additional amount? Why was I told that I need to call in so that I can request the bank to add an additional amount on my debit order amount? Who is going to pay the interest that I normally kill on a monthly basis or the interest I accumulated for January?