South African mobile service provider MTN plans to sue the Nigerian telecoms regulator that fined it $5.2 billion in October over unregistered SIM cards.

The legal action buys MTN more time and keeps the Nigerian Communications Commission from taking any further action if MTN fails to pay the fine by the Dec. 31 deadline, according to IndependentOnline.

Nigeria is MTN’s biggest market by subscribers and revenue. The fine is about 37 percent of MTN’s total revenue and represents more than MTN’s total sales in Nigeria in 2014, BusinessDayLive reported.

MTN, which is valued at $4.672 billion, said it has valid grounds to challenge the amount of the fine and the way it was imposed.

“The manner of the imposition of the fine and the quantum thereof is not in accordance with the NCC’s powers under the Nigerian Communications Act,” MTN said.

The decision to sue was prompted by a “review of circumstances leading to the fine,” MTN said without giving details.

Nigeria asked telcoms operating in the country to verify the identity of their subscribers or cut them off. There are concerns that unregistered SIM cards are used for criminal activity. Nigeria is trying to fight off Islamist militant group Boko Haram.

Some analysts have said the over-the-top fine risked damaging Nigeria’s efforts to shake off its image as a risky frontier market for international investors. Others said the fine showed the government was trying to enforce the law.

The Nigerian Communications Commission reduced MTN’s fine by 25 percent and gave it until the end of year to pay. MTN said it won’t pay.

MTN was in discussions with the Nigerian regulator about how many people had to be cut off when the commission decided to impose the fine on advice from the state security agency, Reuters reported.

Just a week before the fine was announced, MTN was named Most Admired African brand and Most Valuable African brand by Brand Africa based on data from a poll conducted by Geopoll, a leading mobile survey platform with a database of nearly 200 million users in emerging markets, MGAfrica reported.

The fine caused upsheavel in MTN’s top management and operations.

Sifiso Dabengwa resigned as CEO. So did Michael Ikpoki, MTN Nigeria CEO, and Akinwale Goodluck, head of regulatory and corporate affairs.

The search is on for a CEO.

MTN operates in 22 African and Middle East countries.