It's more about objection handling and not letting prospects fall out of their funnel. In email, if someone has an objection and they are an avoidant type (most people), they will ignore the email thread for a long time and potentially forget about the thread completely. This can drag out communications for a weeks as the sales person is constantly having to revive the thread after it is apparent the prospect is avoiding. On a phone call, they can sense the apprehension, elicit the objection and handle it in minutes rather than days of silence. For this reason phone calls feel more efficient and safe to them.