i choose phone calls because it's interactive instead of one sided.
sure, a note is nice, but a phone call allows you the opportunity to express your true feelings through conversation.
my argument still stands: if you want to come off as though you care enough to send a holiday card, sign it. people aren't stupid. they see a fucking card with a stamp on it and say "wow, i'm one of x number of people automatically generated by a database. merry christmas to you too, CHARLES L. WHATEVERYOURNAMEIS"