UPDATE: Regarding the below response I just gave. I just received another appt with an .ics file link, and I can't get it to post to my calendar in any way.
So in the famous word of Emily Litella....."Nevermind."
Let the Microsoft dysfunction continue!
It's now working for me.
With my Outlook calendar open ( open it by right-clicking on the Outlook icon on my toolbar and selecting "Open Calendar")......
I downloaded the .ics file attachment in the email (onto my desktop, so it's easy to find).
I double-clicked on the .ics file icon showing on my desktop.
I left-click on that Outlook icon on my toolbar to have the Outlook calendar appear on my screen.
The calendar appears, with another smaller window prompting me to select a calendar from a drop-down. There's also a button to "import".
(There's a "Browse" button, but in my case, the file name was already filled into the field next to it.)
I only have one calendar, so I select that one and click on the "import".
A message appears at the bottom of that smaller window confirming successful import of the file.
I check my calendar, and lo and behold....the appointment is there.
When it works, and with a little practice, it is faster than manually creating an appointment on my calendar.
But yeah, if someone (like a client) doesn't include the appointment information, ya can't do the manual thing, right?
Anyway, good luck.
I don't know if I just stumbled onto a one-time or sometimes situation, or if I will be able to rely on this working....