How to set up network for Ekiga on Arch Linux?

How to set up network for Ekiga on Arch Linux?

I've got two machines with identical Ekiga versions and settings
(including accounts), both behind the same router. Both have contacts
referencing myself and the standard call back test.
Machine A:
Up to date Ubuntu 13.04
Connected to wireless network
Machine B:
Up to date Arch Linux
Connected to wired network
No port forwarding is set up on the router. On machine A, after logging
in, my "self" contact shows I'm online and I'm able to successfully
complete a call back test with audio and video. After logging out on
machine A and logging in on machine B, I do not see myself as online and
the call back test just opens the call window for a long time and never
calls back. No messages are displayed indicating what is happening. Some
debug output:
$ ekiga -d 4 2>&1 | grep STUN | cut -f 3-
Ekiga Started STUN detector
StunDetect...6b8651d700 OPAL STUN server "stun.ekiga.net" replies Cone
NAT, external IP 91.190.8.175
Housekeepe...6b8c0dd700 Ekiga Stopped STUN detector
MonSock Created bundled UDP socket via STUN,
internal=192.168.1.3:5060,
external=91.190.8.175:5060
I don't get any "PDU is likely too large" messages.
I tried setting up port forwarding to machine B as well. The result is
that machine A complains "Ekiga did not manage to configure your network
settings automatically", but machine B is still not able to do anything.
Despite the warning and the port forwarding setup, machine A is still able
to successfully complete a call back test.
The only difference I could find in the two clients is that A has some
extra video codecs - H263, H263-1998 and MP4V-ES, to be specific.