I stumbled on the same problem even with Asterisk 11.7.0. I think the problem is related to the NAT problem as described in url voip-info.org/wiki/view/IAX (NAT implementation section) - if there’s NAT involved.
Recently, I got an assignment to setup two asterisk servers, both were behind NAT (without VPN on each site), and only using port forward implementation. In some x hours, I got UNREACHABLE status between IAX2 trunk connection, and the only told me something like:
NOTICE chan_iax2.c: Peer ‘eIAX196’ is now UNREACHABLE! Time: 89
Even I got disconnection while call session was in progress and the log said:
WARNING chan_iax2.c: Max retries exceeded to host xxx.xxx.xxx.xxx on IAX2/eIAX196-293 (type = 6, subclass = 2, ts=7
I did a lot of config maneuvers including changing the maxexpiry, defaultexpiry, etc. And I also adding static port setup on both routers to avoid port rewrite, all directions. And still got same problems next few hours.
Problem was solved by implementing SSL/TLS vpn (OpenVPN) on both sides. But still I need to know if I could still connect IAX2s behind NAT without aforementioned problem.