I called a friends cell phone via Gizmo on my N800 with the N800 headset/mic.

He said my voice had a "sci-fi" quality, and I believed his voice broke up a bit.

I had him call me back to my N800 via my Grandcentral # (to try and resolve the screening & answering function).

He said my voice was much clearer and the connection seemed better.

I see on my PC that the Grandcentral site can answer or call back a previous caller. But I haven't been able to properly log in to Grandcentral yet on my N800 to test any call out function.

I don't think it's safe to come to any conclusion from one call. If there's any hard rule, it's that internet telephony is inconsistent.

Judge at least 10 different calls from different locations and I may trust the differences you've found.

Grandcentral uses Level 3 Communications.
It's the same sevice that google uses to connect free goog-411 calls.

When, and if, I can successfully log into GC on my OS'07 N800 I'd like to try the "click2call" option. (the pop up login button seems tricky - w/even less success w/ microb engine).

It's supposed to ring me and the other person - though the ring and call would have to come thru Gizmo, and not the webpage interface. But then I could compare the virtual Gizmo/GC "call out" options on the same friend's phone connection.

If he's calling your GC number, it's still being routed through a third party (whatever service it forwarded the call to). If that was your gizmo number then you are still using their network and would suffer the same poor call quality if it was a recurring issue.