Doubling/Change of billing amount with no notice

I am usually charged R297 per month by ADT. Today I received a notification from my bank that R551 was deducted from my account. Repeated attempts to contact the call centre have resulted in calls being terminated. <br> <br> I was given NO NOTICE about a doubling in charges. I have received no INVOICE for any other charges. Please advise what this charge is for, otherwise I will be going to the bank to dispute this as a fradulent charge on my account.