To link your National Identity Number (NIN) to your MTN phone number, you can follow these steps:

1. Dial *785# on your MTN line
2. Select “NIN Capture” from the menu
3. Enter your NIN and submit
4. Wait for a confirmation message indicating that your NIN has been linked to your MTN line

Alternatively, you can visit an MTN service center or retail outlet and provide your NIN and other required information to have it linked to your MTN line. It is important to note that linking your NIN to your phone number is a mandatory requirement in Nigeria, and failure to do so may result in your line being deactivated.