Ok, I have analyzed logs of Mika sessions and have found that when Mika
set his NAT to map ports, a GTalk on his side refused to send candidate
stanzas with type = "stun" (STUN is the way to pass NAT) during ICE.
The only useful candidate stanza arrived to our voice service bot has
type set to "relay". Sure we did not implement relaying (proxing)
feature yet in our library, that is why Mika's GTalk could not
send/receive media flow. The question rises: why Google's
implementation of STUN/ICE could not pass port-mapped NAT ?