I don't have the AT&T Fiber 20 X 20 so I can't answer to that. Much of good QoS is dependent on the router being used and the settings therein. I obviously don't know which router your customer is using but here is a starting point for you to review with regard to RingCentral QoS issues. There is a listing of routers and other settings for Qos there.
Hi All. Currently we have plenty of customers who are using RingCentral VoIP services off of ATT Fiber... without QoS challenges. And some who have experienced challenges (however have been resolved).
Overall ATT's routing of SIP traffic should be no different than industry standard specifications... at least as of the past 2-3 years. SIP is pretty much a widely accepted (standard) protocol by all who operate in the VoIP world and is routed as real-time/time-sensitive traffic, at least at the access points and/or endpoints.
With that said, the root cause of all QoS challenges is almost always either (and/or a combo of) latency (delay), jitter (variances in the delay), and/or packet loss. This could be due to malfunctions/settings-related in equipment (phone/cables/switches/routers/modem), lack of resources (adequate bandwidth), or network performance-related (LAN/ ISP carrier).
To get to the root cause, we need to understand exactly what the QoS challenges (and other related details around it)...so that it can be more effectively addressed. Again, if you are experiencing QoS challenges, you are also experiencing one or more of the following underlying issue(s) ...latency, jitter, and packet loss.
ATT's Fiber service/connection is acting as a transport to haul the SIP traffic (and also your non-VoIP traffic). If it is not performing as you've provisioned the connection for... i.e. steady/consistent available bandwidth and delay, please do reach out to your ATT representative to get the issue addressed.
Lastly, as best practice, before you get on the phone with ATT, please ensure that you do have adequate resources and/or resources setup on from your LAN on out to the router. That is, enough guaranteed bandwidth to accommodate simultaneous phone calls (100Kbps for each call/both directions; so if have 5 phones being used at the same time, you should have 500Kbps or .5Mbps reserved and prioritized for the phones, both directions). This can be done from within your router... QoS-related feature/functions (if available). Overall this may or may not address your challenges, however, it guarantees the root cause of the challenges is not stemming from your LAN, and is most likely is the ISP equipment/ISP performance, on out.