Early cancellation process

I had a very bad experience with Vodacom, I made an early cancellation and I was provided with a settlement letter, I was so excited that I finally closed my account, and a month later when I checked my account was in arrears, and when I call in I was told that when I was provided the settlement letter, an invoice for that month was already out therefore it was not included on the early cancellation statement, alright then my question was why was that not communicated so that I know and pay the account. Meaning if I didn't check my credit enquiry I wouldn't have known that I'm owing Vodacom. Vodacom communication is very poor, quick to debit and put your account in arrears but no proper communication, I was not provided even with an invoice so that at least I'm made aware that I have an outstanding balance, now my credit score has reduced just because their communication channel is not proper, please improve Vodacom is a big company.