Netstar uses underhand telesales methods!

Someone phoned me and said they were from Netstar and that as my tracking device was about 7 years old I should update it to the newest model. I agreed and a few days later a technician came to m,y house and fitted the new device and gave me a "Fitment Certificate". About a week later I sold my car and phoned Netstar to cancel this arrangement. They informed me that I had agreed to a 3 year contract and I would have to pay 3 years premiums to cancel the contract. The telesales person did NOT mention anything about a 3 year contract and that my monthly debit order would increse from R69.00 per month to over R180.00 a month. I am an 86 year old pensioner who would never agree to a 3 year contract as I will probably be dead before the contract period has expired. I sold my car because I desperately need the money to survive. Nothing was ever given to me in writing other than the "Fitment Certificate". Please help me to get Netstar to cancel this agreement. Vehicle registration number is CA919688