I have a stupid question. I'm in CA. If I get a provider like CallCentric (with servers only in New York) and I call my neighbor, does the traffic go CA -> [VOIP] -> NY -> [PSTN] -> CA ? Or does CC somehow have local POPs so the traffic stays on this coast?
This is how I understand it... When you set up your ATA or softphone, it registers with CallCentric server in New York. VoIP signaling protocol is SIP - it's responsible to connect your device (also known as user agent) with some VoIP-to-PSTN gateway that is probably in your neighbor's phone company office. Which is probably likely somewhere in your neighborhood. So, once this connection is established, actual voice traffic goes over RTP bypassing CallCentric SIP servers.
In most trivial case, when your neighbor is also CallCentric customer, RTP traffic goes directly from your device to his. I am pretty sure that in a little bit less trivial case, where your neighbor has VOIP service from different provider, voice traffic goes directly as well.
Last, this isn't really OBi question - if you are really curious, you may want to ask in VoIP forum (http://www.dslreports.com/forum/voip) on DSL Reports.
Callcentric proxies all audio, even for Callcentric->Callcentric calls. Your audio is routed via New York.