contract running over stipulated duration

I took out a data contract with Vodacom in 2016 which came with a laptop. I have been paying for that contract since 2016 July and when I check the next date for upgrade I was told 01/05/2019 and cancellation date for the contract is 01/08/2019. can someone explain to me why the contract is now going to run for 37 instead of 24 months as your call centre is absolutely useless and no one can explain to me what the problem is. I refuse to continue paying further until someone calls me and provides me with a proper explanation all your invoices show that I took out a 1BG 24 months top up contract so how you going to extend this without my knowledge?