Like it says in the subject line, really:

When I try to call someone (actually, I have only one contact in my Google Talk contacts list; how sad am I, eh?) with the 770's gtalk applet, it starts the call, displays "calling" first, then "connecting..." and then does nothing for almost a minute and disconnects, displaying a call time of 00:00 (which is correct, as there was no call). Oh yeah, the presence icon is a very pretty green colour all the time.

Calling the same person on the desktop and on the same wireless network, gives no problem at all: we can chat for hours on end.

I have no idea what I'm doing wrong.