I'd assume if I'm making a schedule for joint event with a second party, I can inform them about the ways I want to get in contact, directly - say, via email. Third party, which helps scheduling - that is, reminding and showing the calendar in advance - doesn't really need to know anything about me, except that I reserved some time slot in the calendar. Granted, that third party would not be able to facilitate the contact itself, but that's the price for keeping PII, well, private.