I just tried a 770 to 770 Google Talk session and it worked. The call was a bit choppy, but not bad considering this is Beta and the call was from Berlin to Toronto. (so internet latency and lag may have played an issue)

I had tried previously with someone on a dial-up connection and it did not work. Perhaps it will not initiate the call if both parties do not have a decent amount of bandwidth?

Just a thought...
-Sandstorm