Best Plugins For Vacation / On Call Scheduling

I am currently building out projects for mobile and web apps for a client in the hopes of turning one multisite shortly.

I would like to build a stand alone Vacation and On Call scheduling app on a subdomain of the core site so I can link to regional sites which are part of same cluster.

Should I be going Appointment route for availability or scheduling for locking down times? Was hoping to be able to generate a calendar file for iOS or Outlook appt for Office.

Any thoughts or suggestions would be greatly appreciated.

Cheers,

Phil