You can already start calls to Alexa devices from Amazon's tablets, but it's now possible to receive those calls, too. As of December 5th, people with recent Fire HD 8 (2017 and 2018) and Fire HD 10 models can take Drop-In calls and intercom-style Announcements when they're in Show Mode. This will mainly be useful if you have a Show Mode charging dock, which invokes the mode automatically. If you're in that position, though, it could be very helpful -- you could tell your kids it's time for dinner without buying another Echo or (gasp) walking across the house.
The feature is rolling out now as a software update. You can opt in or out of both Drop-Ins and Announcements in settings, so you shouldn't have to worry about surprise calls on your bedroom tablet. You can also limit calls to specific contacts if you only want to talk to your closest friends and family members.