Trying to settle my debt.

I fell behind on my Vodacom Contract. I have been trying to make an arrangement for day's now, using the *** number. Keep on loosing connection. today I finally got through, requesting to cancel my contract which lapses tomorrow and allow me to pay off the outstanding the amount. They say they cannot do that before the outstanding amount has been fully payed, and they are going to invoice me the full contract fees until I am fully payed. I even offered to pay them R500 a month, which they say cannot work because I will then never be able to settle the outstanding amount, which will rise by R1200 per month. How can this be legal?