ACCOUNT WRONGLY LOAD AND NOT AS PER CONTRACT SIGNED

My son got a new contract in December and the conditions was when the airtime and data is finished we can upload airtime/data. The monthly account was paid as per the contract amount. In July my son wanted to upgrade and he was then notified that he is in arrears with R3000 as there was overspend on the new contract of December. We were no were notified of the number that is overspend or a bill was received. We immediately query and said impossible as we load airtime/data and why we were not notified. My son got several query numbers and MTN confirmed by call that it was the branch mistake as he signed for a closed contract and the contract was open. They will escalate the matter to the relevant department and send us the contract. How is it possible after 7 months when my son wants to upgrade all of a sudden he owes them money, never informed him. When his phones were 1 day late with payment they cut it. On several occasions did he contact MTN due to late payment and no were nobody told him there is an arrears outstanding. Once his phones were paid up to date the phones was on or not cut off. He is not happy about the matter and MTN just say it is his fault he must pay or they will cut the lines. ***, ***, ***, ***