Teeki, I think is fine over phone I have also done by email. The important thing is to notify the method is not important IMO. DH has also done over the phone. Email for me was easy as I was in another country (twice have done this way both in permanent positions). It isn't like breaking up with someone where over phone is a no no. If you were in the office and manager at next desk obviously better in person but as not then phone or email fine.