LAGOS, Nigeria, Muharram 14, 1437, October 27, 2015, SPA -- MTN Group telecommunication's stock plunged 12.5 percent on the Johannesburg Stock Exchange Monday on the news that its most lucrative subsidiary, in Nigeria, has been fined $5.2 billion for failing to disconnect millions of unregistered cellphone subscribers, according to AP. The Nigerian Communications Commission confirmed the penalty based on 200,000 naira ($1,000) for each of 5.1 million cellphone SIM cards that had not been registered and should have been disconnected by an August deadline. On Aug. 7, the commission issued an ultimatum ordering all cellphone providers to de-activate unregistered cards within seven days "or face severe sanctions." All the other cellphone companies in Nigeria complied, the commission said. The South African company said in a statement Monday that it is in talks with Nigeria's regulatory body "to resolve the matter." The $5.2 billion fine is equivalent to at least two average years' profit for MTN Nigeria and nearly three times the $1.8 billion that it has invested in the West African country, according to the company's website. Africa's leading cellphone service provider, MTN paid $285 million for one of four GSM licenses in Nigeria in 2001. Before the cut, MTN said it had about 62 million customers among Nigeria's 170 million people in September, by far the group's biggest market. Nigeria has about 150 million active mobile phone lines, 90 million mobile Internet users and imports 4 million cellphones each month, according to Nigeria's Federal Ministry of Communications. --SPA 00:19 LOCAL TIME 21:19 GMT www.spa.gov.sa/w