Johannesburg/Lagos - Africa's biggest mobile phone operator, MTN, is in talks with Nigerian regulators to try to reduce a $5.2 billion fine, two sources familiar with the matter said on Tuesday. The Nigerian Communications Commission (NCC) said on Monday it had imposed the penalty on the South African-based firm for failing to disconnect users with unregistered SIM cards. "MTN is talking to NCC with the view to revoking or reducing this fine," one source said. "There was bit of misunderstanding around this issue." Another source said MTN had been in talks with NCC about the exact number of people using unregistered SIM cards when the deadline to disconnect them passed. "MTN was under the presumption that it can carry on business as usual because it was still in discussion with the regulators," the source said. If it stands, the penalty will wipe out more than two years of MTN's annual profits. Nigeria is MTN's biggest market by subscribers.