I believe that post you linked to is outdated, Voipo does proxy audio now on all calls. In the case of sip7 the audio proxy shows as 72.52.231.45 (Liquidweb Michigan) and in the case of sip and sip-central01 it shows 67.228.190.147 (Softlayer Dallas). So this does have an effect in the call quality, at least for me.