mtn making my father to pay R18 000

my father missed three instalment on his mtn contract but after when his financial difficulty changes he pay that R3000 that he own including areas but a week after that he got a call from people saying that they are from mtn lawyers and that he must pay R18000,even when he settle his outstanding amount and the mtn even send a letter after that saying he should pay R600 and I when with him to mtn store in Brooklyn mall the say he own R389 but got sms saying that the lawyers are taking him to court, how can mtn make my father pay R18000 for the contract that is ending next year in April , the amount that is more than the cost of his phones and his not using the airtime he phones are blocked , mtn is taking my father for a ride, my father didn't even create a contract with mtn but nausha mobile &amp