Continued charge for cancelled contract

I have been a loyal customer of Telkom's for more than 5 years with numerous contracts. When my data contract expired, I was contacted by the Call Centre to offer me a new contract with a new Router at the price I was currently paying. What the caller did not tell me, was that they were offering me a 10G contract, when in fact I always had a 10G + 10G contract. I went to the Hemingways branch to change the contract to a 20G +20G contract and cancel the 10G contract. Completed a cancellation form and was assured that the contract would be cancelled. The courier collected the unused and unopened router the following day. Imagine my surprise when I received my statement and I am still being billed for the contract that was cancelled for January and February. Went back into the Hemingways branch who told me even though I signed the cancellation form, they won't cancel the contract until the Retentions dept have contacted me. Conveniently for them, Retentions dept have NOT contacted me, despite numerous calls to 081180, visits to the branch etc. SURELY A DOCUMENT IN WRITING SHOULD SUFFICE AS A CANCELLATION???