Send them a registered letter asking them to prove the ammount owed, date of the last payment and that they are authorized to collect the ammount owed. This is called Debt Validation, in Quebec they have 10 days to respond to this sort of request or the debt collection is assumed to be illegal as they dont have this information.
I dont know if the same 10 day rule applies in Ontario but perhaps someone else can answer this. Call the number on this site and ask what the reules are in this case to be sure
Ministry of Government Services: Services for Individuals