Billing of cancelled services and threatening credit messages

I had to very bad experiences with billing: First, Matrix Telematics sent me a couple of SMSs in December charging a service that was paid on the first day of the month. On top of that, I received threatening messages that I'd loose credit due to that. After waiting for hours on the telephone, it was explained that it was a message sent to all the clients due to the festive season. I don't get what's different during this period of the year to receive such messages. The second issue was a debit of a canceled service that it's now impossible to talk to somebody over the phone, as there's no customer service available to fix their own mess on the financial department.