If you call back the offering agent with more questions, and the agent goes super nova on you, that is very important information to have.
Remember, the agent is in the business of communication and negotiation. If she can't communicate and negotiate with you, who she wants to sign an agreement with, how is she going to handle communicating with editors who don't necessarily want to sign the book she's offering?
Now, don't call every five minutes or else you're giving the agent very important information about you too. But if you email with, "I have a few follow up questions, so can we please schedule a call?" and then you have a brief, professional conversation at the appointed time -- that should be fine. Also fine if you email with, "I have a few more questions. Would you prefer I email them to you, or can we have another call? It would only take about fifteen minutes."
As for not doing the call right -- yeah, of course none of us does the call right on the first go-around. How much experience do you have landing a literary agent? How much experience do most of this agent's authors have before their debut? It's fine. Part of an agent's job is guiding your career, and they know you're at the start of it.
Ideally you'll be working together for years. It's okay to ask for another fifteen minutes.
