We reviewed the ROAV VIVA device by Anker a while back, and actually still use it constantly for mobile Alexa capabilities. It’s a brilliant device. At the time of the review, it supported all Alexa features except Calling and Messaging. Most non-Amazon devices suffered the same drawback, but the company opened up its API recently allowing third-parties to also take advantage of this feature. The device would allow calling, but it used the cell phone connection instead of Alexa device-to-device support.
A recent update to the software and a firmware update for the device has given it the ability to call other Alexa devices or to drop-in to your Alexa devices at home. This makes a great device even better, giving it the full breadth of capability and making it arguably the best in-car Alexa device.
The device is still available from Amazon.com for $50: Roav Viva by Anker