Unauthorised contract upgrade

I took up a cellphone and data contract with Vodacom in 2014 and i have since left south africa on Nov 2014. I was only waiting for the contract to expire so i can cancel it. I went back to South Africa in October 2016 and called vodacom line to try and cancel i was told i still has three months to go, i thought it was odd so i left it. Now i am trying to cancell the contract and i am told that i have upgraded the contract and it will only expire in September 2018. So you tell me why will i upgrade a contract for a number o dont even use? Please Vodacom cancel my contract as i want to close my bank account.