Incompetence at its best

I got a sms saying I owe discovery R2208.36 as they had paid more than the savings I had from my previous plan. I was not made aware of this prior to the cancellation and reactivation on the new plan, no sms, no phone call, and no email to advice of this. I called discovery and spoke to a Winnie who claims my previous employer was advised of this!! What I don't understand Is WHY advice a company that I'm no longer with of my issues/ debts? Why now send \me"the sms that I now owe money. I didn't leave discovery I just downgraded the plan. This just show that the agents who handled my request are ********"