Is it appropriate to phone a researcher/academic? (Germany)
I don't appreciate being called; it imposes your time schedule on me. Others differ. However, I suspect that email is your safest bet.

The practical problem is that many universities in Germany have been closed due to COVID-19, and the numbers listed will be the work numbers. Either you are calling an empty office, or your call is forwarded to the persons home adress, which may not be what you want.

@MassimoOrtolano makes a good point in emphasizing the unannounced part of a phone call. You could sent an email to make an appointment. I would be fine with that.