loyal customer disappointed

I have phoned vodacom on 3 different days and my issue has still not been resolved and I still haven't gotten an answer. I initially got my contract in 2016, previously it was on my grandfathers name. I upgraded June 2018, on my contract it says 24 months but vodacom has so far told me I can only upgrade May 2021, but none of these people can tell me why, the first person told me it was because i took a 3 year contract which is not true because I have the contract and it says 24 months. What is happening here Vodacom? You are taking 11 more months' money from me?! I am very dissapointed since I have always been a loyal vodacom customer.