I would tend to agree. My intuition is also that (1) some headhunters might use time-spent-on-the-phone as a metric linked to bonuses (think: click as a proxy for sales in online advertising) and (2) the phone lets headhunter gather intel that you end up leaking in the discussion and that you would not leak via email. Some have great tactics to make you speak “now that you’re on the call”.