I admire your capacity to extend basic charity to confirmed Outlook users. I'm not sure they're the demographic any software project ought to establish as a primary test market.
If one is using roundcube, by definition one is sitting at a networked computer and ought to have access to any of the myriad online calendars out there. Ditto addressbooks. For addresses and phone numbers I also use Yahoo, but in a mail app all I really need and want in addressbook form are . . . email addresses.
Best,