Wrongfully handed over to the legal team

I paid all my outstanding debts on the 29th of March 2021, but today (15 April 2021) I've just received an sms stating that my account has been handed over to SSDA. Why is this so? I have tried to reach out to Cell debt collectors through the email ***, as it was earlier communicated to me that I should use it, but the email address doesn't exist.