Sim card delivered to wrong person! Someone has been using it for more than 8 months

have received the worst service from Mondo. I got into an MTN contract with them for a device. I was sent the wrong Sim card and I have been trying to get assistance for almost 10 months now, they only called me for a collection. After explaining to the lady from the collection Dept, she said she'd call but till today no call. All I wanted was clarity as to who is responsible for the huge mistake. I have been paying all this time and someone else was using the airtime and data I was supposed to receive. I did not pay for 2 months because I wanted it to get resolved but nex. Mondo is nonsense!