Africa’s largest mobile operator, MTN Group,  said on Monday that the Nigerian regulator has fined it $5.2 billion for failing to disconnect unregistered subscribers in the Africa’s biggest economy.

The Nigerian Communication Commission (NCC) fined MTN Nigeria about $1,000 for each of the 5.1 million SIM card under its network that were yet to be registered even after a set deadline for doing so ended in September.

In reaction, MTN shares plunged as much as 14.25 percent to 166.99 rands a share at the Johannesburg Stock Exchange, according to Reuters data.

“We had a directive that was given to all the networks to deactivate all unregistered SIM card on their networks and they had enough time to do that,” Reuters quoted Tony Ojobo NCC’s director of public affairs saying.

He added that all other mobile operators in Nigeria complied with the directive, apart from MTN.

“We came out with all guidelines and the fine was supposed to be 200,000 naira ($1,000) per SIM card,” Ojobo said.

MTN said in a statement that it was in discussion with NCC to “resolve the matter”.

The operator’s subscriber base in Nigeria, which stand at about 62.5 million, has been on the decline in recent months mainly due to stiff competition and tough regulations, a recent quarterly report showed.

Statistics from the NCC indicates that the West African nation of an estimated 170 million people, has almost 150 million mobile phone subscribers.

Analysts said the NCC directive was heavy-handed and could cost MTN Nigeria at least two average annual profit and nearly three times the $1.8 billion the telecom company invested in the West African  country.

“It’s an extremely punitive fine,” Dobek Pater of Africa Analysis told Quartz Africa.

“The regulator has approached the matter in a quite heavy-handed way, but there seems to be no outright objection from MTN in the announcement of the fine. I think MTN will opt to enter into negotiations with the NCC.”