What's important to you about your (potential) job?
Interviewing needs to be a two way street. The employer has skills/qualifications they need, and they have a culture they need to fit you into. The employer knows these factors, and chooses their questions based on them.
Similarly, you have career aspirations, and skills you need to be compensated for. You may also have very practical concerns: work hours, dress code, etc. You can't decide what questions to ask until you know what you're looking for. We can give you ideas, but ultimately the questions will be wasted unless they're meaningful to you.
The practical questions (hours, pay, dress, etc) are probably the easiest to think up.
Career path questions may take a little more thought. How do you see this internship fitting into your overall career path? Frame your questions around that. Assuming this internship represents a stepping stone into your desired career, and you like this employer, you may want to ask things like,
- How often do you convert internships into full time positions?
- How mobile are employees within your organization?
- What support do you have internally for career development? Are there tuition subsidy programs for supporting further education? Leadership development programs? Support for certifications or professional training?
You mentioned "culture" which is important but hard to speak of in generic terms. If you ask a generic question like, "what's the culture like?" you will probably get a similarly generic answer. There are more specific questions which can paint a picture of the culture:
- What's a typical work day like for most people in my position? A mix of meetings? desk work? lots of interruptions, or longer periods focused on a single task without interruption?
- How do employees get feedback on performance?
- Do employees typically share work and collaborate openly, or are employees more focused on their own tasks?
- Are interns actively mentored?
Finally, it's worth pointing out that you've been asked to a 15 minute phone interview. That's very quick - come up with a list of questions, but think carefully beforehand which ones are most appropriate for a short conversation, and which ones you care the most about. Others can be asked in a second interview (if there is one) or through whatever channel you have into the company assuming things keep progressing - ie a recruiter, etc.
Based on your edit, which clarified that you are concerned over the specific fact that this is for an internship, I would still emphasize the same factors as above - decide what is important to you, personally, and ask questions about those specifics. If you want to grow fast, ask questions about growth opportunity. If you want to try different departments, ask questions about mobility. If you want to immediately pursue additional education, ask about tuition assistance. etc.