Create required classes, enums, etc. for the given requirements. Assume we need to represent employees, each employee has an id, name, phone number, designation, manager, department and team members working under him/her. The manager is also an employee and team members are also employee (definitely, multiple members in a team). There are only four departments (a fix list, you can say) e.g. SALES, PURCHASE, ACCOUNTS and WAREHOUSE. We also want to store phone number of each department (assume it would not change).

The employee phone number shall be composed of area code and line number. Define Employee in such a way that its object should not be created unless user provide id, designation and the deparment. (Only classes/enums definitions are required, no need to create objects and get any data from user to demostrate complete usage etc.)