I’ve also ended interviews as a candidate on the phone myself. They asked a question that I didn’t’ have the answer to and I told them that I didn’t know. The next 2 questions were in that same direction, with them knowing that I’d already said I wasn’t particularly fluent in that area but they kept on. At that point I said, “Let’s just stop here. We both know that I’m not doing well answering your questions and to be honest, that you’re restating the same topic after being told that already I don’t know probably means we wouldn’t be a good fit.” Too many people forget that it’s a two-way street and they seems shocked that anyone would actually end their interview.