Old overdue amount

I received an sms from ADT on 19 May 2015: ADT - Dear client, your account reflects an old overdue amount of R530.10 which will be taken via debit order on 29.5.2015.? For queries please contact Simon - ***<br> <br> I have never been made aware of this overdue amount, they have not informed me how it arose, and I have been on debit order with ADT since I signed up with them about 13 years ago.<br> <br> I called the number provided, but Simon was on his tea break. I left a message with Eugenia for him to call me back when his tea break ends at 3.pm. He didn't call back. I called the number a few times after 3.15pm, and no one answered. <br> <br> I have emailed ADT at their inland email address. The auto-response is that they are experiencing high volumes and will get back to me me. I sent them a complaint about something else a few weeks ago, and they never got back to me. I don't expect them to respond this time either.<br> <br> I believe that it is not right, nor legal, to inform me of a deduction that they have not justified, or discussed with me.<br>