Billed 3 years after contract expiry

I terminated a cell phone contract with Virgin mobile in May 2012. Three years later I received a letter from a company called MBD collections informing me that they are collecting on behalf of Virgin mobile an amount of R100 and failure to pay will result in me being responsible for interest. The problem I have is that it has taken Virgin mobile three years to inform me that I owe them a May 2012. As per my recollection I paid all amounts owed to the company because they had a debit order with my bank but then again it was 3 years ago how will I be able to recall what happened? And on top of that I no longer bank there! I called the collections company and they referred me to Virgin Mobile. I called Virgin mobile demanding them to inform me why they did not contact me a week, or even a year after the contract ended to inform me there is an outstanding payment. They had my contact details yet it took three years. A lady at collections called Lerato has now referred me back to MBD collections to enquire why they did not contact me yet she also informed me Virgin Mobile handed over the account in 2014. I demand answers on why I am contacted in 2015 this is outright unprofessional..