Upgrade vs Cancelation of contract

Their upgrade department has been calling me everyday telling me about upgrading. I have been refusing to take their upgrades telling them that I want to cancel the contract. No one ever mentioned to me that I have to write or call them if I want to cancel the contract. now they are sending me an invoice whereas I have told many of their call centre agents that I don't want upgrade I want to cancel. they must take back their airtime because I cannot afford to pay them. Their call centre can record when they have called me for upgrade, but they can not record that I told them I want to cancel the contract. They must just stop this daylight ******ies. They even trained their call centre agents to threaten their customers that the ACCOUNT DEPARTMENT WILL DEAL WITH THEM if they don't pay AND NOT FORGETING TO MENTION THAT THEY CAN NOT HELP THEIR CUSTOMERS TO CANCEL CONTRACTS. if they don't help us, then who should? Why do they help us to enter into this robing contracts where they don't even explain the terms and conditions clearly? I also requested for the letter confirming that my contract has ended and the lady who was helping me told me that they don't give out any letters.