Until now, you could only initiate those calls, not receive them. Announcements let you use your tablet as a one-way intercom, which is handy for blasting a message to the rest of the household if you have Alexa speakers in other rooms.
You'll need one of the most recent Fire HD 8 (2017 and 2018) or Fire HD 10 tablets to make use of these new features. You'll also probably want to get yourself a Show Mode charging dock to keep the tablet stood upright.
The new feature is rolling out to Fire tablets right now, via a software update. You can choose to opt in or opt out in the settings, and even better, limit the features to select contacts - just your favorites.
To set it up, head to Settings in the Fire tablet's drop-down menu and then Alexa, and ensure Hands-Free Mode is on. You'll then want to go to Communications and toggle Calling and Messaging to on.
Finally, toggle Drop In and My Household to on (for other Alexa devices on your account), then select Announcements and toggle it to Enabled.