MTN Group Ltd says it is reviewing its senior management in Nigeria after Africa’s largest phone company was fined $5.2 billion by Nigerian Communications Commission (NCC), the sector’s regulator, for failing to disconnect customers with unregistered SIM cards, according to a person familiar with the matter, reports Bloomberg.

Senior executives at the company, who are in Nigeria to meet with the regulator to discuss the fine, will decide on whether to make changes to senior management, which is headed by Chief Executive Officer, Michael Ikpoki, said the person, who asked not to be identified because the matter is private.

Ikpoki didn’t respond to calls and messages seeking comment.

Chris Maroleng, MTN’s main spokesman at its head office in Johannesburg, declined to comment.

“We know of no such thing,” Chineze Gbenga-Oluwatoye, a spokeswoman for MTN Nigeria, said by phone from Lagos.

MTN shares have plunged about 19 percent this week in Johannesburg, the biggest three-day drop since 2008, valuing the company at about 288 billion rand ($21 billion). The phone operator said on Monday the Nigerian Communications Commission is seeking the penalties because it missed a deadline to disconnect 5.1 million subscribers.

The stock declined 2.6 percent to 155.85 rand at the close on Wednesday, the lowest since October 2012.

The fine is based on N200,000 ($1,004) for each customer. Nigeria is MTN’s biggest market, with about 62 million customers at the end of September.