Supplies with 280 V @ 75 Hz are illegal in Germany in both home and work place, with the possible exception of laboratories decorated with large blinking warning signs and access restriction to people in long white lab coats.
In general, employers and employees do not have the same level of power or strength. Because of this inbalance, at least in Europe, there is no unlimited "freedom of contract". Even if an employee signs a policy, not everything would be valid. About this specific case, the whiping of a private telephone, I assume it would be illegal in Germany, even if the employee has signed the policy. But before there is a real case and a court takes a decision, we cannot know.