Totally annoyed

Last November someone contacted me (I was under the impression it was someone from Vodacom not an outside company) to upgrade my phone, which at the time I was not interested in doing, and she said that I was eligible for a discount on my calls as my old contact had now lapsed. I was not aware that I was agreeing to a whole new contact. I only became aware of it when the "new contact" fee was deducted from my bank account in Dec, when I tried to query this I was pushed from pillar to post and nothing was resolved. I tried again today, as I now want to upgrade my phone and have been told by an incredibly rude woman (at Mondo) that I am locked in to said contact because I agreed to it over the phone. At no point was I ever sent any paperwork via email or any other means as a reference for this contract so I have no way of disputing what I may or may not have agreed to. Because of Mondo this whole experience has soured my relationship with Vodacom and in future will be moving to a new provider.