MTN security number is a unique 16 digit pin issued to you to prevent third parties from making transactions on your account.

What are the benefits?

1. It will protect you from Mobile Fraud which results from third party transactions.
2. Makes it easier for you to carry out sensitive services such as MTN SIM Swap, PUK number retrieval, PIN Reset, Share and Sell, Call Divert and more.
3. Gives you full access to online self help channels via MTN Nigeria’s Web Portal.

How to Get your MTN Security Number

Just walk into any MTN Walk-in center or call MTN customer care on 180. You may be asked your PUK Number, Amount of Last Recharge, Residential Address etc to verify that you are the owner of the SIM. Once your SIM is verified you will be sent your security number.

Hope this was useful. You can get more details from the link below.

Source: http://www..net/mtn-security-number-7078/ (Nigeria's Leading Technology Blog!)