My income tax assessment explains that in future I will have to provide evidence of my professional telephone costs over a period of three months if I want to deduct more than 20% of the total telephone costs as income-related expenses. Is this correct and what kind of evidence should I provide?

If an employee incurs telephone expenses for work-related reasons, up to 20% of the invoice amount,
a maximum of € 20 per month, can be recognised as income-related expenses for reasons of simplification without individual proof. However, if you wish to claim higher expenses, you must provide detailed evidence of the share of the expenses incurred for professional reasons in the total expenses for a representative period of three months. The professional share determined in this way can then be used for the entire year. The
telephone bill with itemisation showing the professional calls is sufficient as proof. If the calls are not listed individually in the telephone bill, detailed records of the calls made must also be kept.