To link your NIN to your Airtel phone number, follow the steps below:

Dial *121# on the Airtel line you wish to link..
Reply by Type 1 for “NIN Capture”
You will be requested to type 1 again to enter your government-approved NIN Number
Enter your 11-digit NIN and send.
Wait for the message confirming your submission. If you get an error, try again, until successful
Airtel Subscribers can also link online by visiting https://www.airtel.com.ng/nin ,

Enter Mobile Number and click SEND OTP
Type the OTP in the space below to confirm your number
Thereafter enter your 11 Digit NIN and submit

Link NIN to Phone Number GLO Subscribers

Compose an SMS with the following lines
*UPDATENIN (space) NIN (space) FIRSTNAME (space) LASTNAME” to 109
e.g. UPDATENIN 23456988389 Peter Yakubu
Send to 109


Source : https://www.naijabusiness.com.ng/news/2020/how-to-link-nin-to-phone-number-on-airtel-and-glo-network/