I think you've both been a bit silly.
It sounds like the person you spoke to either didn't have the info to answer your question or didn't have the authority to give it out (i.e. you got put through to someone who just does telephone screening). Maybe they didn't have the confidence to simply say so, or didn't listen to your question.
However, it also sounds like you weren't anywhere near assertive enough in the conversation. You needed to recognise that you weren't getting the information you were after and that there was a one-way flow of information taking place. If you were being asked questions that you hadn't prepared for, you should simply have said that you were only calling up for preliminary information and weren't interested in interviewing for the position until you knew more.
For similar calls in future, you can prepare more. Beforehand, think about what information you are after specifically so that you are in a position to provide direction for the conversation. The person you speak to won't automatically know what information you are interested in, so you effectively need to interview them. If you call a recruitment agency, remember that they are primarily interested in identifying good candidates for the job, so they need to actively qualify or disqualify you from the process in order to determine whether it is worth spending a lot of time discussing the role with you.
You mention that you didn't really understand the role to start with, which to on the face of it doesn't appear a great starting point. However, you obviously saw something in the role description that interested you and where you saw a match with your skills and experience, otherwise you presumably wouldn't have made the call. You can use that as your starting point - sell your strengths against that parts of the role you do understand in order to get the other person's interest and assure them that you are a worthwhile candidate (rather than a timewaster). From there, you can probe into the areas that aren't as clear to you.
This approach applies equally whether you're calling an agency, company HR or a hiring manager directly as they all have limited time to spend chatting. If anything, you need to be more focussed when calling a company directly as they will be taking much of the role description for granted and will not have a large stock of alternative roles for which they can consider you - your telephone sales pitch will therefore be all the more important.