To add to other ansers, an interview in Germany is often only a opptortunity for the applicant for becoming asked to finally visit the group a half/full day and for the employer to filter out interesting students, when too much good ones apply. In my field, physics, >100 applicants for good PhD positions will be normal. Maybe 3-4 will be invited to visit the group actually and then the professor or a committee makes the decision. If you're living in Germany, they will often simply invite you to make a presentation at the face.
It's a fast and important opportunity to check if you can fluently speak the language (at least English) or rule out eventual contradictions/gaps in your CV or ask some informal questions (scientific/engineering family, hobbies,...). But, from my experience phone interviews are not very common in Germany for STEM PhD positions, also not in non-academia job world.
I don't have so much experience how overseas applicants are treated here as we don't have many and master degree is often necessary (you didn't say how far away you're from this group), if interesting candidates are even invited to visit groups and traveling costs are paid (at least partly). But traveling within Europe via train to visit the group is common and costs paid. Mostly after 1-2 mails without a pre-phone-interview, because a well written CV is often enough to invite then 3-4 good applicants and fill the position. If you are applying for a very specialized topic from overseas where it's unlikely that there are many good candidates, then I would take this much more seriously and prepare really some text to present your master work in a short and pregnant way. Some interviewer will expect this or tell you explicitly, but in any case it is your fault to be not prepared :)
So I would check and was checked myself language proficiency, short descprition of master's work, relation/motivation to/for this PhD work and country, other applications I filed and what I plan to do after finishing PhD. You will maybe also face some hidden questions. There was one position I was interested in where it became known to me after interview and visiting the group (but finally not offered) that the professor searched somebody spending also some post-doc years at this group (a small young group that was just build up) and not ruling this out completely. Such questions some professors will ask you explicitly, some indirect, as it is hard nowadays to forecast your life 5-10 years. But showing the professor that I just wanted to do PhD in his group and city and then move on finally cost me this position.
Formally it's an interview of you, nonetheless you should also ask some questions at the end of the interview that are not mentioned in the job description (salary, teaching duties (language demands, master students), advisor, overall time frame, attending conferences, vacation...). It's not a good strategy to bombard employer in mails with such questions before it is not clear that they are really interested in you. Show that you're are thinking determined self-confident curious guy, a bit like a good flirt ;)