Too quick to remind us about payments but can’t send paid up letters when accounts have been settled!

I had an account with MTN which was handed over to MBD and I’ve since paid up the account in February this year. After which I’ve been requesting a paid up letter but I’m not being assisted. They keep telling me that they’ll send the letter via email but it’s been 4 months now NOTHING. The last agent I spoke to a few days ago told me that she’ll send the paid up letter within 24 hours and it’s been more than that. What should one do to get a paid up letter from MBD? They used to harass me every month day before my payment date which they can a reminder to pay but as soon as I finished and now requesting a paid up letter from their side, they’re not assisting.