Vodacom has changed my contract expiry date

I signed up with Vodacom on a 2Gb data contract in January 2016. this was to be a 24 month contract. On contacting Vodacom in November 2017 to place notice of my intention to terminate the contract at the end of January 2018, I was advised that the contract is only due for termination in March 2018. Vodacom has failed to explained to me how a 24 month contract which commenced in January 2016 has to expire in March 2018. I am only allowed to communicate with the call centre operators and my requested to have personnel at managerial contact me has failed. Vodacom is in breach of this contract.