To check the registration status of your MTN SIM, send REG as a message to 789.
After sending the text to 789 with your MTN line, you will get a reply confirming that you have been registered, like:
+23480XXXXXXX has been registered. Thank you!
Note: +23480XXXXXXX is your phone number!
Alternatively, you could call the MTN customer care on 180 to confirm your SIM registration status