If I never want to experience any voice breakage or jitters in using GV, I just use my landline to call my GV number then dial out from there. When doing that, the call is pretty much perfect. Why is that the case Steve?
I guess it depends on what sort of land line service you're using -- copper wireline POTS, or FiOS fiber optic POTS, or SIP VoIP. If that leg of the connection is high-quality, then it may be more reliable than your OBi <--> Google Chat leg, for the network quality reasons mentioned earlier.
If you eliminate or ignore any contribution to poor connection quality by your own ISP, and only look at the path from GV outward to the called party, it can be going over very fat pipes between Google's backbone and a large-scale carrier via a direct peering relationship, or it could be hopping through GV to a carrier, to that carrier's transit carrier, to the final carrier, before it hits the called party. It all depends on which other carriers are involved "from here to there".
An interesting side note, is that, if you eliminate the PSTN carriers, or at least eliminate their low-bandwidth POTS-era design, and instead make calls directly between two VoIP endpoints, it opens up near-future dramatic improvements in call audio quality through the use of adaptive multi-rate wideband audio CODECs, such as G722.2 AMR-WB. The good news is that the mobile phone carriers are rapidly deploying this ("HD-Voice" over LTE), a few VoIP carriers are supporting, or will support it, and Google Hangouts supports it already.