Now what I do find interesting is this is a Android device, so if you can install apps (you can) you should be able to install SfB and then you'd have SfB on the phone as well.
It would likely work as a secondary phone service though that is in no way associated with the SIP service you have. FreePBX etc
Using something like SfB on it might go beyond its intended use... using it for that would save on cost of buying Polycom, but if cost and quality was a concern then the client wouldn't use SfB at all.
Wondering if this solution is a weird little niche where Ubiquiti can just check a box that says "we offer VoIP".
Well this isn't a VoIP offering, its another device that takes advantage of the VoIP capabilities. These don't work without a phone system.
From the documentation I found, it looked like the USG would be able to control the devices; but I haven't looked into exactly what extent or if they are just devices with an Internet connection and an Android phone app.