Cell billed me for more than i signed off,

Good day, My name is Hilda, i received a contact from cell c to contract myselfl on device for phone for +- R250 plus insurance for device. I was billed R744 instead of R250, i called and query on mail, the agent said i have two contract , data sim and phone sim , i am using 074 they said i got 06... for which i never use it. i asked to cancel to sim card which i am currently billed R500, FOR NOT AWARE. I got fibre i dont need data sim. The agent said i must pay R3500 to cancel sim. Hello peter, i never used the sim card, i am asking to cancel and stick to original contract of R250 PLUS INSURANCE. The ref is ***/ cell ***