Number belongs to Someone

Hi I bought my son a cellphone that come standard with a sim card and because he doesnt have contractual capacity it was Rica'd in my name . We have later found out that the number belongs to someone as we get messages and calls of his creditors and it seems like he was subscribed for few games . Currently when he buys airtime it goes to those subcriptions . I would like MTN to either give us the new simcard which doesnt belong to anyone , refund our airtime and we cant have creditors harassing us with someone we dont know . What MTN did was wrong in every level and I want my Son's airtime to be refunded .