I've been trying to use audio chat with google talk account in empathy (ubuntu 10.10) without success.

regular text chat works normally.

but when I "call" a contact and he answers - it immediately gives him disconnected message while I get connecting or connected message and call duration counter (depends on the settings).

but when a contact is calling me and I answer - he keeps hearing ringing and I get disconnected message and this error:
Can't establish audio stream
Can't establish a connection to XX. One of you might be on a network that does not allow direct connections.
Technical Details
Could not establish connection
in windows xp, on the same computer, google talk audio chat works without problems...

I've already tried all of the suggestions that I found using google and searching this forum (updates, gstreamer and other codecs, google account setting etc.).
what else can I try to do so I can use google talk in ubuntu?