What code can I use to check my mtn phone number using serial code on another mobile

Which code can I use to check my unregistered MTN number

I want to know link my nin card to my mtn line but it say that I have already link another nin card to it. How can I link my personal Nin to this mtn number...

To get the MTN Number, go to the phone contact list and check for number. MTN Number is automatically saved in your contact list for some days after the first activation.