poor service

good day I have bought cellphone (iphone) on contract with mtn two years ago. The contract ended this year on January. since then my phone was giving me problem of not reading the sim card. i went to mtn stores they cant help , their respond was that mtn can not block the phone so there is nothing they can assist me with. today i went to iphone store to check my phone they say mtn has blacklisted my phone , they have showned me the system to see myself that the phone is blacklisted and they adviced me to go the nearest mtn . I went to mtn again they cant assist me. i have bought expensive phone now is not operating