Bad experience and worst Custer treatment

They asked me to upgrade my phone when my contract was about to end I agreed but they never delivered the phone but increased my premiums. I reversed my premium after my contract end and the asked me to pay and they'll cancellation my month to month contract. I paid them and got over 5 SMSes of cancellation request received 3 months later they say I own 3 months premiums cause I didn't send email of cancellation.