More than full charge when contract was cancelled via email 2 months ago

My contract was set to expire in April 2026. I wrote a cancelation email on 28 February 2026 to cancel my contract when it expires. No one rep**** or contacted me. Only today when they sent me a message via WhatsApp to renew my contract they told me to cancel by calling 08 135. The now want to charge me more than I paid for the phone for another month. This is daylight ******* charging me the amount for the phone again with the data/airtime. Inexcusable, I will not refer mtn to anyone.