Asking me to pay for your incompetence

I asked for month to month data contract which i was told i could cancel at anytime. I called in today to cancel the month to month data allocation due to financial difficulties and now i get told that i pay for cancellation because it was actually loaded as a 24 month contract but i never asked for 24 months i asked for a month to month. I asked whoever i spoke to on the phone to investigate and listen to recordings, if he has to and he was never bothered, he kept on asking me to pay. I ask for a month to month and id like for it to please be cancelled with immediate effect, you cannot ask me to pay for someone else incompetence.