Account submitted to debt collectors even though closed and settled for a year!

We settled and closed our ADT account more than a year ago. More than three weeks ago, I received an sms stating that my account still shows an outstanding balance and that it has been forwarded to Norman Bissett & Associates (Debt recovery service) for collection! I phoned ADT on that day, speaking to Alethea Kannemeyer who said that she would submit it to accounts and that they would come back to me. I have since contacted Alethea via email, imploring her to contact me in regards to this mistake and how ADT would go about fixing this, but even though I have been sending emails constantly, I have heard nothing back about rectifying this mistake. It has been three weeks now. I found it VERY upsetting that my account which has been paid in full, has been submitted - with the possible repercussion of blacklisting me! I also find it upsetting that I received an sms about this. No one phoned either my partner or myself about this 'outstanding' amount. It was just done.