Someone upgraded my contract.

I took out a laptop contract about 5 years sgo. This was a 3 year contract. When the contract was up the device payment came off the bill and I carried on paying the R94 a month thinking one day I would upgrade. I see now that from either December or January someone upgraded this contract and has already raked up a bull of about R6000. If I didn't upgrade this contract then how is it possible that someone did. *****. Please help.