The government banned outgoing calls from unregistered numbers from April 4.

Around 51 million MTN Nigeria subscribers have linked their identification numbers to their SIM cards, CEO Karl Toriola told an analyst call.

The country's telecoms regulator had in December 2020 ordered mobile phone providers to add identification numbers (NINs) - containing personal data identifying the user - to every SIM card registered in the country or block the SIMs.

The government had extended the registration deadline several times before the latest directive to bar calls from unregistered SIMs.

The MTN group CFO said Nigeria is its biggest market, contributing about 35% of group revenues, and the blocking of unregistered numbers could have a small impact on group revenues.

Toriola said a flare up in the security situation in Nigeria after last month's train attack in Kaduna, northwest Nigeria, could have triggered the government's decision.

The government did not give a reason for the timing but the policy is part of its attempts to boost security amid an Islamist insurgency and a spate of kidnappings.

(Reporting by Chijioke Ohuocha;Additional reporting Nqobile Dludla in Johannesburg; Editing by Kirsten Donovan)