Account Status and Paid Up Letter Nightmare

I settled my account on 10/02/2018 which was the amount that I was given by a Call Center Agent, a week later I received an sms from EBC that I had short paid the amount that I was given and need to settle the rest of the account otherwise my account will be taken to legal department. I contacted EBC and was told the person dealing with the account is on lunch but will call me back, lunchtime, this person never returned my call and I phoned back and the call centre agent advised me to ignore the sms which I did and asked for my paid up letter once more which I still haven't received. Yesterday again i received another sms stating that there is no record of my overdue payment and proof of payment has to be resent again which was sent on Monday. I mean this is incompetency, how many times must one ask for a paid up letter just to get all sorts of excuses all the time, am very frustrated that no one may actually be able to solve this query and tired of sending the proof of payment, please sort this out and send me my paid up letter