Phone upgraded with approval...locked into Vodacom contract I did not request

I have been a Vodacom customer for over 10 years, but I have never been more dissatisfied with a service provider in any sector than I am now. I have a cellular contract that has been upgraded without my knowledge or consent, and therefore I am locked into a two-year plan that I do not need nor want. My previous contract was due for upgrade in February 2017. I received a call form Vodacom, or a agent calling on their behalf to discuss possible upgrade options. I clearly stated that my device was in good working order and therefore did not need an upgraded device, but wold like the option do so in the future when I was ready. I was offered a migration that would decrease my monthly contract cost, because my device was paid off, and was told I can upgrade my contract and device at my convenience. In July I visited a Vodacom store to get a new device and upgrade my contract, but was told that a upgrade had been done on my behalf. The implication was that I would be unable to upgrade for another 18 months. I initiated a process to contest the contract via an official affidavit which I submitted to Vodacom. I have emailed and attempted to resolve this since the first week of August yet I have not had a single response from Vodacom.