German trade union Verdi called a nationwide strike on Sunday at Deutsche Telekom's customer service department as negotiations continue over pay and conditions.
The strikes would continue on Monday, Verdi said. More negotiations between the union and the telecoms firm are set for Monday and Tuesday in Potsdam near Berlin.
A demonstration is also planned for Monday, with 4,500 employees expected to attend.
Verdi is demanding a pay increase of 12% for around 70,000 employees, with a minimum raise of €400 ($432) per month.
Telekom recently offered to increase salaries by 4.2% from 2025, with a tax-free inflation compensation bonus this year of €2,000. Verdi rejected the offer as insufficient.