MTN causes stress

Good day,

I lost my Phone on Saturday the 11 July 2020, Please note that i have had the phone since 2016. Since then i have moved places 5 times. I contacted MTN and they said i should go to a branch. when i got to the branch, they said they want the IMEI number. I dont know where i can get this IMEI number because i dont even know where the box is of the phone is now. I checked my monthly invoices but i am unable to find this number. My partner checked his monthly Vodacom and Telkom invoices and this number is indicated there. Why is MTN so behind? and expecting people to keep boxes for years and they can not get the number from their system still puzzles me. I need my phone black listed and i don't know what to do because there so much information in my phone that i am scared that if someone gets the phone they might even use my Bank Account.