*996# - To check if your line has been linked to your NIN
*346# - To check your NIN
*785*your NIN# - To link NIN to MTN
*109*your NIN# - To link NIN to GLO
*200*8# - To link NIN to 9mobile
*121*1# - To link NIN to Airtel

*For internet service providers like Spectranet, Smile, etc, kindly contact their customer service centers.
**If you just linked your line to NIN, kindly wait for at least 24 business hours. If it's not yet linked afterwards, kindly contact your service providers' customer service center and obtain a ticket I.D for your complaint.
***If you service provider has not helped you after you've done the above, kindly report them to Nigerian Communications Commission by dialling 622 and obtain a ticket I.D for your complaint.

If you have questions, you may ask in the comments.

PS: I won't answer the same question twice.