Same issue:
On incoming teams calls, user can hear the ring but no 'Answer' dialog appears. After the calling party hangs up, user can see the missed call. I checked for system health notices in the Office365 admin center; there were no relevant notices.
Windows 10
Teams -> Settings -> Calls:
- Calls ring me: checked
- If unanswered: do nothing
System > Notifications
- Microsoft Teams: On
- Show notification banners: checked
- Show notifications in notification center: checked
- Play a sound when notification arrives: on
- Tried priority Normal and High, neither worked.
- Show notification banners: checked