Due to the rising theft and fraudulent activities, one must protect what he/she has, and the target of swindlers are now majorly your sim cards.
With your sim, anyone could get access to your sim saved numbers, OTP request, password reset, Bank fund transfer, Bank airtime purchase, ATM card pin change, BVN information, NIN information etc.

To protect yourself, do these:

1. Sim lock. First you must change the default pin of your sim cards to new one. The default pin for each network,
MTN : 00000
Airtel: 1111
Glo : 0000
9mobile : 0000

If your sim is locked during the attempt or you forgot your PIN, use the PUK code from the sim pack to unlock it or you could call customer service on 300 for all networks and you will be provided new code after you have proved ownership.

2. Whenever you carried out sim swap or sim upgrade, destroy the chip(the metal part of the sim). Most times the sim is collected, you can still lock the sim before giving it out after upgrade or swapping.
Anyone could still get the sim card and use it to retrieve information when you forgot or refused to destroy it.

Note:
1. If you cannot carryout out the sim lock yourself, get someone to show you how to do it and set easy PIN, then later change the PIN to another.
2. There are different phones and the steps involved in sim lock are different, so you could get someone to assist you or use google.

Source: https://tarferret..com/2023/08/lock-your-sim-cards-now.html