poor billing

I had an account with virgin mobile in 2011 it happened that I lost my phone during that year(2011)and I contacted virgin mobile to cancel the contract as I no longer had the phone they agreed to do that but to my surprise virgin mobile kept on deducting money from my account for +-6 months I contacted them again to close that account and they stopped deducting, after 3 years a company by the name of MBD LCON calls me and says I must pay R100 towards my virgin mobile account as it is overdue by R3900 ,my question is where do these so called MBD comes from? why did they not notify me in 2011 that im still owing? and lastly they must remember that a debt that was not payed for more than three years automatically falls away