Vodacom *****s you money

Have been having this problem with vodacom for some time, 2 years ago I upgraded my phone a few months before the time, having to pay back both phones that I understand paying both the first time, not this is the secont time I upgraded, now I dont understand why I have to pay more than 24 months, according to what I have paid, the phone I got last year was payed off, why do I have to pay like 5 extra months on the phone, regardless what the contract time is, the contract stipulated that the phones payment will be completed in 24 moths, not more, this is *******, vodacom will be losing me as a customer after this.