Google voice app does not use VoIP, it used a cellular voice call to connect to google voice servers. Quality is much better but might not fit with what you want to accomplish.
Skype uses SILK or g729 codec which work much better over 3G than G711. E.g. if skype does not work well over 3G then you will probably not find a VoIP solution that does.
I can use G711 over t-moble 3G (good connection) but skype sounds better.
You did not say which provider you are using, your cellular data connection sucks but so do most cellular data connections.