When I use GV call back app what is the network being used to make voice calls?
GV is not a VoIP service, it uses your network and your minutes.
When it calls be back and I'm connected to call the voice call is on AT&T network?
Unless your phone can connect to one of the other carriers (doubtful), then yes. It's not like GV moves your phone over to TMobile for its call...
Reason I ask is because quality of call although good enough for voice doesn't seem as good quality as making call direct. I thought maybe it's possible on a different frequency or something.
It definitely is running through some sort of VoIP network at some segment of the call. The increased latency is very noticeable.
VoIP?