First, thank you everyone for the very helpful replies! (Sorry for the delay, I forgot to Watch this thread.)
The RG is a residential gateway. Basically a wireless router with extra functionality so they can monitor your connection status to provide some support and troubleshooting. It's also rated for full Gbps speed.
The adtran converts the fiber to a RJ45. It comes free with your fiber.
So you can actually use your current router to save $10, but you'd lose support, telemetry, and that service insurance. Also no guarantee that your current router is fast enough to support Gb speeds.
I think what I'll do is accept install of an RG, then I'll have a baseline of stability and speed against which to compare using my own router instead. Worth a $10 monthly charge or two.
IIRC a phone line comes with a Sonic install. (Which I have zero interest in.)
Are these RG devices also providing the phone output jack?