Calendly has a feature whereby the booker's phone number is shown front and center on a calendar invite/event, whereas TidyCal seems to be centric on the notion that a booker is going to be calling our number rather than we calling them.
As such, we have to set up a custom field to capture the booker's phone number, but seeing as we don't have the ability to customize the calendar event, we have to go back and dig into emails to find the number we need to call.