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

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