That was a somewhat puzzling announcement, as I would think that the market for this is small and getting smaller.
The device is a modified Grandstream HT800 series ATA. It looks like they're only using its FXO port to turn it into a landline gateway. The concept is: someone calls your landline number (e.g. POTS or some VoIP provider), and you've connected the box to that service via an RJ-11 cord. The inbound call is then somehow bridged to your Echo device(s). I guess it would be attractive to people who have landlines, and are too lazy to answer the call on a regular telephone handset or speakerphone, but it seems like a solution for a problem that doesn't need solving.
I successfully made a couple of calls to 10-digit numbers yesterday on one of my Echo Dots. The call quality was quite good, although the Dot's tiny speaker is no comparison to a decent speakerphone.