Changing the terms of the contract during the contract term

When I took out this contract at Woodlands Telkom in Pretoria I have been told the following: On the Unlimited Call and Data package I will be able to call any local Telkom number and local cell phone numbers EXCEPT 0800, 086 and 087 numbers without any additional charges. I took the contract on the above terms. Telkom changed the terms of my contract before the end of the 24 months, saying that any business number is no longer part of the unlimited call package. ********** address given by call centre, but the email does not work to lodge your compliant with Telkom Mobile. It gets returned undelivered. Client’s cannot reach me on my cell phone, when my clients attempt to contact me from any business number (Standard 012 – Telkom landline) then they hear a voice message stating that ‘This number is no longer available on the MTN network’. My clients are now irritated and frustrated with me because they are unable to reach me. When I called the call centre on 22 August 2017 at 11:52 – 12:05 I have spoken to Maxine Petersen who informed me that I, as a paying customer of TELKOM MOBILE have to take the matter up with MTN? This is 100% unacceptable as I am no longer a paying customer of MTN, but have been paying TELKOM MOBILE for my cell phone services. Telkom Mobile has to sort the matter out. I am unable to call business Standard 012 Telkom landline numbers. I am using this number my cell phone for my business, since 26 July 2017 I am unable to call all business Telkom landline numbers for example my head office in Secunda 017 638 ---- or a client 012 991 ----. Does anybody know the contact details of a Telecommunication regulator in South Africa?