Telkom SA, the South African telecommunications company, fell 30% in Johannesburg after the company warned it was considering an “impairment of assets charge” of about 13 billion rand ($679 million).

Significant market changes and economic conditions, including low expected growth rates, have hurt the group, the company said in a statement on Wednesday. Telkom, which announced plans to cut as much as 15% of jobs as part of a restructuring plan in February, said it had extended the voluntary severance packages to all of its employees.