Cell C changing terms of contract

At 7am this morning, I received a call from VVM attorneys and they told me that they are doing collections for Cell C. I told them that my account with Cell C is up to date, I don't understand why they are calling me. The lady told me that from now going forward all Cell C customers will make payments to them and not Cell C. Whether the account is up to date or not, Cell C has contracted them for this service. Why did Cell C not inform us clients about these changes. I have a contract with Cell C not VVM attorneys. If Cell C is changing the terms of the contract, should they not inform their clients first. I am concerned about the annoying calls from these attorney collection firms that makes you look like a bad payer. I am so unhappy with this arrangement.