Why am I still being billed after cancellation

I want to know why am I receiving a massage for an early payment when I have cancelled my subscription with you. The last time I had a conversation with your agent, he told me that I will be cancelled and I will no nolonger have any bill, to pay.. then why am I receiving a massage regarding payment of a sim that I cancelled and stopped using ..I cancelled it on the 15th of June and your team said cancellation will be done on the 1st of July.while I wanted an immediate cancellation. After being told that the subscription will be cancelled and I will no longer need to worry about the bill, why am I still being billed