Promotions not for full 24 months

I am having an issue with one of my contracts. I took the contract in January 2017 and my 24 months is over in January why would MTN stop the promotion on the 23rd month but will expect full payment for all 24 months. What I can't understand is that the promotion was on black friday 2016 people that took it on that day had the benefits untill end of November 2018. I took the same promotion in January 2017 but my benefits was already stopped. I tried calling the call centre but had to hold 20min as there is no supervisor available and now I have to phone again in 3 to 4 working days to find out what the outcome is of my complaint. So this just cause customers too loose out even more.