Like Tom Squires, I only have experience in conducting phone interviews for smaller companies. But yes, it is a screening interview, which means it really is going to boil down to "no, don't proceed any further with this candidate", or "yes, get this candidate in for an interview".
Obviously there are degrees from "no" to "HELL NO!" and from "yes" to "yes get them in RIGHT NOW!!", but ultimately it's a filter.
And in my experience (this could well be different at larger companies), it is indeed all in the hands of the phone interviewer, without any particular discussion of the interview. The goal of the phone interview is to filter out unsuitable candidates with the minimum amount of time and effort, which does pretty much mean one person needs to take care of it, not a group.
Objective or subjective measurements? I'd like to think it would be more objective than a full interview. I'd prepare some questions based on the candidate's resume and previous work, and if they couldn't answer them to a satisfactory degree, that would probably make the outcome a "no". But I'd be reluctant to rule someone out based on "vibe", I'd prefer to leave that sort of judgement to a face-to-face interview with multiple interviewers.