Using Outlook for iOS for personal email and calendar management
Looks like this feature was intentionally removed.
I got the following response from Support: "As per checking, this is a recent change in the app for privacy reasons. This is now the intended behaviour in the app. For now, you can submit a request for this feature through Settings > Help & Feedback > Suggest a feature."
As JohnEdmondsonmentions above, the workaround is to place the dial-in in the description of the calendar invite and the dial-in works like before.