You can find general "starter" questions from articles like 51 Great Questions to Ask in an Interview. I recommend you to prepare 3 to 5 such starter questions. However, be mindful that most applicants also look up articles like that and it does not make you distinctive.
The more you can prepare personally relevant (to the person) questions, the more flattered they'll be, the more likeable you'll seem, and the more effective the call will be.
Beyond starter questions, most important is for you to listen actively, stay present, and be curious during the call.
Once they give an answer to your original question, ask follow up questions, like,
- What was that like?
- How do you feel about it?
- What's an example of that?
- Would you say that's the general case?
Some other starter questions
- I saw that you previously worked at [Previous Company], which is a company I'm also interested in. What would you say are the similarities and differences between [Previous Company] and [Current Company]?
- You wrote XXX in your profile. I found that really interesting. Can you share more about that? [XXX is something interesting in their profile]
- For hiring managers: What's been your greatest challenge so far in hiring for this role?
From "How to Become Insanely Well-Connected"
- What are the key milestones in their career?
- What expertise do they seemingly love to provide? (Possible to suss out from any articles that quote them or talks they’ve given in the past.)
- Are there any recent news stories or announcements about them?
- What do you want to ask them or get out of the interaction?