Customarily, phone calls are used when the callers need to have the responses as soon as possible, or when they don't receive replies by other channels for whatever reasons. Receiving an important call at an inconvenient situation might create a bad impression on the receiver. However, for a recruiter who wants to offer a candidate a job position, when do they decide to call instead of emailing?
I think it's best for them to stick to email. Emails give both parties time to think and write, while phone calls usually end up with "thanks for the info, let me think about that", then emails will be used afterwards. An answer of Do recruiters call twice to candidates? said that "recruiters who are truly interested in getting a response will [...] likely e-mail you rather than call", but it doesn't give any explanation. The only positive side of using phone calls I can think of is to making a pressure on the receivers, but I don't think they really want to do that.
So when should recruiters use phone call instead of email?