Telecom giant MTN Nigeria has been fined a record $5.2bn by the Nigerian Communications Commission (NCC).
MTN was fined for non-compliance with a deadline set by the NCC to disconnect all non-registered SIM cards.
The move follows accusations by mobile phone users that the regulator had failed to bring operators to account for poor services to subscribers.
MTN Nigeria says it is studying the letter sent to it by the regulator and will respond soon.
MTN is one of the largest phone providers in Nigeria.