In line with the new policy of the National communication commission (NCC) that all phone number should be linked with national identity number of this owners.

The National Identification Number (NIN) is the unique number that identifies you as a Nigerian and is issued to you at the completion of enrolment into the National Identity Database (NIDB) by NIMC.

NCC had threatened that failure to do so would lead to people’s line being blocked or disconnected by December 31, 2020.

https://ogbongetech.com/2020/12/how-to-link-your-nin-to-your-9mobile-line/