Unacceptable

I took 3 cellphones on contract and we agreed on contract that the debit order will run for 24 moths. they went into my account for a few months and they decided to stop. 3 years later I get a call telling me that I owe MTN R10 000. R10 000??? got to be crazy. How is it my fault that you stopped going into my account and never even told me that you are going to do that? and they 3 years later you tell me that I owe you so much much money? that is just insane. I feel that MTN should have contacted me when they decided to stop going into my account than coming 3years later telling me that I owe you so much money.