Bandwidth.com used to be the underlying carrier for GV but I believe the last time I checked it was Level 3, maybe it depends on the number. I can check numbers if anyone is interested using a basic telecom tool I have access to for work. (I work for the 4th largest carrier in the country). Level 3 is a major league carrier, but all this means is that they are terminating the call on the pstn or public switched telephone network, or even more basic converting it from voip to "standard" analog.
I am a GV (sprint integration) user and I have noticed call quality issues as well. I have also perceived a delay in getting ring-back, it seems like the calls take longer to set up. I believe that Sprint phones are actually dialing into some type of server, delivering a terminating number to that server and then waiting for the server to make a call to the final terminating number and then that server bridges the two together.
Likely that server is owned by Google. Google is then converting calls to voip and then delivering them in packet form to Level 3 who does the rest. It is possible that this is not accurate, but I bet it is.