MTN Nigeria has been fined $5.2 billion for failing to disconnect 5.1 million unregistered cellphone subscribers.

The stock of the South African-based company fell 14 percent on the Johannesburg Stock Exchange on Monday’s news.

The Nigerian Communications Commission confirmed the penalty based on 200,000 naira ($1,000) for each cellphone SIM card that had not been registered by its August deadline.

The Nigerian Communications Commission said all other telecom firms operating in the country complied with the directive except  MTN, Africa’s biggest mobile phone company, Tony Ojobo said.

The regulator said MTN, whose biggest market is Nigeria, has not denied the infraction and that its enforcement team deactivated 5.2 million unregistered lines to ensure compliance.

MTN promised a statement later Monday.

The $5.2 billion is at least two average years’ profit for MTN Nigeria, by far the MTN Group’s most lucrative subsidiary, and nearly three times the $1.8 billion that it has invested in Nigeria, according to the company’s website.

MTN says it has about 62 million customers among Nigeria’s 170 million people.