Nigeria has been rolling out 11-digit electronic national identity cards for almost a decade, which record an individual's personal and biometric data, including fingerprints and photo. The National Identity Number (NIN) is required to open a bank account, apply for a driver's license, vote, get health insurance, and file tax returns. Officials have said the policy is needed to protect against kidnappings.

Constance Chioma calls her son every morning to check that he is safe while studying in northeast Nigeria, a region plagued by deadly attacks by Islamist insurgents and armed kidnappings.

Earlier this month, she could not get through.

She later realised her SIM card was one of about 73 million – more than a third of the 198 million in Nigeria – which have been barred from making outgoing calls because they have not been registered in the national digital identity database.