I see... Yes, really is a bit confusing. Especially knowing that not everyone had to do that. For example, those two girls from my AD...no such email, just a quick call to ppl they've nominated.
After reading your post just now, I've texted them both to ask did anyone try to contact their referees via email before calling them. They both said no and they said referees were literally called one after another.
I wonder why some are contacted by phone, some by email. I would understand email in a case of a big time zone difference, such as mine, but the rest would just be easier calling. Oh, who knows! We just need to be patient and hope all goes well.
Has it been a long time since you received that first email? Fingers crossed for you!