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Does that mean that voice calls between the audio only models (ie Echo and Echo Dots) is a forthcoming added feature for them?
Yes... anyone with any model of Echo product, and/or anyone with the Alexa app installed on an Android or iOS device, will be able to call anyone else with a device or the app. Obviously, the non-video devices will just have audio calling. One of the thoughtful features (likely to be panned by people too lazy to think about its value) will be "Drop in", where the user can authorize a very few, select people to initiate a call with them without the called device needing to take action to answer. This would be a good tool for checking in on an elderly person in another location.
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Amazon taking on Skype?
Sort of. The press is interpreting the calling and messaging feature as an added-value extra to the overall Echo/Alexa ecosystem, for now at least -- no PSTN calling or SIP URI calling announced. Amazon also bought a teleconferencing company recently, and it is incorporating their software into an Amazon-branded conference calling solution. It seems to me that Amazon would have a steep path to climb to reach feature parity with Skype, Facebook Messenger, and Hangouts, though...and, would they want the very large cost of setting up and running a VoIP/PSTN external network like Google Voice? It's amusing that the Echo Show is just another generation's idea of the early 1960s Bell System PicturePhone!
The US tech press is all over this, so you can lose a couple hours of your life, like I did this morning, reading about it. I have three Echo Dots spread over my two-story house, mainly used for home automation via voice control. One article pointed out that the calling/messaging feature is a logical development that is finally easy and robust to deploy, thanks to widespread high-speed internet service, low-cost hardware, and modern A/V CODECs.