I have a very bad experience with MTN, their agent sold me a data card on contract and told me the card has 10G in it and yet it was just 2G data

I have a very bad experience with MTN, their agent sold me a data card on contract and told me the card has 10G in it and yet it was just 2G data. I returned the card the following day and cancelled their contract at MTN branch EL. What upsets me is that they are sending debt collectors to call me every day, demanding payment from me and yet I never used their data.