Nigeria on Monday sent the continent’s telecommunications market in shock with its surprise $5.2 billion fine on Africa’s biggest mobile network, MTN, for failing to disconnect unregistered subscribers.

The fine saw MTN’s stocks at the Johannesburg Stock Exchange (JSE) plummet by 12 per cent, the biggest single day decline since 1998 as investors quickly offloaded their units.

The Nigerian Communications Commission’s fine, which is one of the heaviest ever laid on any African operator, puts the firm’s investments on the continent in question even as analysts say MTN will take many years to settle the penalty.

Tony Ojobo, head of public affairs at NCC, told Bloomberg that they had switched off the 5.2 million subscribers, which was MTN’s responsibility.

NCC accused the firm of delaying the disconnection of 5.1 million MTN Nigeria subscribers between August and September, who operated unregistered sim-cards.

MTN in a market update confirmed that it is facing the fine imposed by NCC on its Nigerian unit.

“This fine relates to the timing of the disconnection of 5.1 million MTN Nigeria subscribers who were disconnected in August and September and is based on a fine of $1,000 for each unregistered subscriber,” said the company.

Early this month, the firm was again caught in another tax evasion storm after being accused of repatriating millions of dollars earned in African countries including Ghana, Uganda, Ivory Coast and Nigeria to offshore accounts in Mauritius.

READ: MTN caught in web of court cases over tax

The NCC has given MTN until November 16 to pay the fine, or face the consequences. The commission said that MTN has in the last few years engaged in 28 infractions forcing the regulator to suspend all regulatory services.

It still isn’t clear what consequences MTN stands to face. The telecommunication firm has seen its shares slump by 20 per cent (Friday morning) even as ratings agency Fitch adjusted its credit rating to a negative outlook.

“MTN complies with all regulations and laws, including tax laws, in the countries where we operate, which include South Africa, Nigeria, Uganda and Ghana.” The firm said in response to the accusations.

In 2013, the firm’s unit in Uganda was accused of making false declarations in a bid to evade $35 million in Customs duty due on imports of network equipment for the company’s Uganda operation. The case was, however, dropped.

MTN, which is valued at $23 billion, further said it is in discussions with the NCC to resolve the matter.

According to Bloomberg Index, the telecommunications firms stocks lost more than 12 per cent, with shares worth more than $363 million trading on Tuesday at the JSE. In its interim results for the period ended June 30, MTN said its subscribers grew by 3.4 per cent in the Middle East and Africa.

In 2014, the firm reported a 1.1 per cent decline its Nigeria revenue due to a challenging operating environment.

African telecommunications authorities have been clamping down on unregistered sim-cards in order to reduce crime and mobile money fraud. Since 2012, Kenya, Uganda, Rwanda and South Africa among other countries that have disconnected unregistered sim cards within their jurisdictions.

Last month, communications regulators from Uganda, Rwanda and South Sudan met in Nairobi to establish the legal and technical framework for harmonising sim card registration in the region. Kenya has already outlawed the sale of pre-activated sim cards while Uganda, Rwanda and South Sudan are at different stages in legislating and implementing sim registration.

Kenya’s Communications Authority director-general Francis Wangusi said that the law is clear on the penalties of either using unregistered sim-cards or not switching of such cards.

Currently, telecom operators in Kenya who fail to comply are liable to a fine not exceeding $50,000. On the other hand, subscribers found using an unregistered sim card are liable to a fine of $3,000 while mobile operators’ agents found to have sold unregistered sim cards are liable to a fine of $5,000. So far no one has been fined or jailed, as all operators are said to have complied.

Uganda is yet to impose any fines on operators with unregistered cards on their networks even as it works on the legal and regulatory framework that will allow the linking of the sim card registration and its national ID database.

The acting director for broadcasting at Uganda’s Communication Authority, Fred Otunnu, said that they will soon announce the appropriate sanctions against offending operators as provided for under the relevant law.