STUN support in Asterisk 1.6 …
… is seriously broken. See also bug 17622. Hopefully Asterisk 1.8 will come with much improved STUN support.
Asterisk 1.8: The ‘stunaddr’ option has been removed from chan_sip. This feature did not behave as expected, had no correct use case, and was not RFC compliant. The removal of this feature will hopefully be followed by a correct RFC compliant STUN implementation in chan_sip in the future. However, 1.8 comes with the addition of the ‘subscribe_network_change’ option for turning on and off res_stun_monitor module support in chan_sip.
See also