Following the instruction given by the NCC to Telecommunication companies to deactivate all pre-registered SIM card, MTN Nigeria deactivated some sim cards where some were not, which resulted to their been Fined by the telecommunication commission.
Those customers whose sim cards were deactivated were instructed to visit any Telecommunication outlet to register their sim cards or update their data.
While some of those whose sim cards were not disconnected are being prompted to visit any mobile outlet to register their sim cards to avoid being disconnected.
Mtn Nigeria is currently compensating and rewarding those who gets their sim card registered with thousands of airtime and other freebies.
If you've not registered yours, you are advised to do so, so as to avoid been disconnected, also if you are not sure whether your is registered or not, you can simply dial *789# on your Mtn sim card to comfirm.
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For other networks, check the links below

>> http://www.mobilitaria.com/2015/09/sim-registration-check-code-for-mtn-glo-etisalat-and-airtel.html