The AOK also offers the option of voluntary insurance. The voluntary insurance is for persons that are no longer covered by a statutory insurance or family insurance, for example. This is the case, should an employee's income exceed the annual earning limit (2018 59,400 Euro). These people then are able to join the voluntary health insurance of the AOK within a period of three months.
The income is used to calculate the contributions of the voluntary insurance. The AOK will gladly inform you.