Pathetic service

I have been an MTN client for as long as i can remember, 2years ago my cellphone got ****** at work and i called MTN to black list the phone, i through the box away as it served no purpose in keeping the box. MTN keeps record of all information for at least 5years that's a Legal requirement, i called the call center for assistance by no one can assist me. i got a Samsung phone recently and realized that the phone i blacklisted is linked to my account, meaning someone is using the phone that is suppose to be blacklisted. No one at MTN can assist me cause i dont have the box, if MTN is my provide why dont they have the details of the phone as it was a contract phone, im disgraced and disgusted in this matter, u promised when a phone is blacklisted no one can use it, so why is it that someone else is using the phone? *** is the sim currently in my phone