The chief executive of South Africa-based mobile phone operator MTN has resigned over a $5.2 billion fine imposed on the company in Nigeria, the telecoms giant said in a statement yesterday. MTN has appointed non-executive Chairman Phuthuma Nhleko as executive chairman for a maximum of six months while the company identifies a successor for Dabengwa, a statement said. The Nigerian Communications Commission (NCC) fined Africa's largest telecoms firm for missing a deadline to deactivate 5.1 million unregistered SIM cards. The Johannesburg Stock Exchange (JSE) has launched an investigation into MTN for "possible insider trading" before the company announced it had been hit by the fine. The probe by the JSE could result in South Africa's bourse operator slapping MTN with another hefty fine or result in criminal charges. Nigeria, Africa's most populous country, is MTN group's largest market where it had over 62.8 million subscribers by the second quarter of this year.