Unilateral change of terms and condition of contract!

Telkom decided unilaterally to change the terms and conditions of my data contract from R199 to R495 p/m. I reported the matter at the Centurion mall store, without ANY success. After I raised the issue with Telkom, on 1 October 2017, and asked them to provide me with either my verbal or written consent to amend the contract, on numerous occasions, and which they have failed to do to this day they instructed debt collectors to harass me and to list me on the credit bureau! This is ******** and bullying of a loyal Telkom customer.