Are you the same person who posted this exact same question a few days ago?
Anyway, the answer is going to be heavily dependent on what country you're from. Since you didn't state that in this post or in the other one, we don't know.
You don't need a visa if you are from the EEA or if you are from:
2. Citizens from countries not requiring a visa
If you belong to a country mentioned below you do not need a visa but only a valid passport.
Republic of Korea
United States of America
And finally, based on some cursory searching, it would seem that PhD students need student "Visum zu Studienzwecken“, as you have already applied for. This is different in some countries (I needed a French scientist visa as a PhD student in France).
The source for my reply re: Germany https://www.uni-hohenheim.de/uploads/media/Visa_and_Residence_Permit_Germany_31.pdf and https://www.daad.de/deutschland/nach-deutschland/bewerbung/en/9199-visa-application/ and https://www.study-in.de/en/plan-your-studies/requirements/visa-and-residence-permit_26604.php.
In France, I was told that I could apply for a student visa or a scientist visa, although the scientist visa was technically the correct one. I would imagine that if you applied for a scientist visa, you wouldn't be necessarily turned away. However since you are already applying for a student visa, and that appears to be the correct route, you're fine anyway.
Finally, the people you should really be asking (rather than us) will be the people at your local German embassy, followed by the human resources/admin people at your Institution. If there is conflict between these two opinions, always go with the embassy once you have a confirmation of correctness from more than one person there.