Can't reverse a transaction

I accidentally settled the outstanding amount on my iPhone - via the FNB App. I had no intention of doing this early - as there is no way I can afford it. I called to cancel the transaction and apparently this can't be done. It happened at about 7a.m and I called to cancel at 8a.m. I don't see how this problem cannot be rectified? It was an honest mistake - that was way too easy to make. I didn't even have to provide a One Time Pin. <br> <br> I now have to max out my credit card for the month to make up for the money that went into my phone and then pay that off (with interest) as well as pay the early settlement fee. I understand I should have been more careful, but surely FNB can reverse a transaction?