The Nigerian unit of MTN Group Ltd, Africa’s biggest wireless carrier, said in a statement on Saturday that it wasn’t accused of any wrongdoing and is cooperating with the investigation.

The South African company listed its shares on the Nigerian Stock Exchange on May 16 to comply with terms of a US$1.5 billion settlement in 2015 with the Nigerian Communications Commission over unregistered phone lines on its network.

Since the listing, the stock has jumped 41%, closing at 140 naira (US$0.39) per share on Friday, giving the company a market capitalisation of 2.8 trillion naira (US$7.7 billion), second only to Dangote Cement Plc.

The Lagos-based bourse said on May 21 that MTN’s shares weren’t available for buyers and urged the company to provide more of the stock for investors.

MTN is also in court challenging a US$2 billion bill for alleged back taxes.