Bad service

I have a contract with MTN, I took a Zlip4 which the contract is still running. Lately we encounter the problem with the phone that when you close it, it switch off. I spoke to MTN for this 3 days trying to sort of the matter as I have to submit it for repairs. I went to the store and information they are looking for for them to take in the phone I don’t have but since I took the contract online, MTN online must provide such information. When I speak to them ( MTN online ), they told me that they don’t have information I need and I can’t take in my phone . Samsung also told me that when I called them that information is needed and they hat MTN is sending is not what they need or want. What am I to do as I am still paying for this contract and it’s up to date but when there are issues that need them to help, it’s pillar to post kind of a thing.