Yesterday, I was unable to connect to any IPv4 SIP providers as well as IPv4 SIP peers. My network was configured correctly and the peers and providers reply to the ICMP echo request. Connections to IPv6 providers and peers works good (for all my work I use only IPv6). My computers runs GNU/Linux Debian Squeeze ("upgraded" daily). FYI on Windows it runs fine.
I found the reason and a workaround. It is due to the introduction of bindv6only=1 by default in GNU/Linux Debian Squeeze. You could check it at /proc/sys/net/ipv6/bindv6only and /etc/sysctl.d/bindv6only.conf. See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=560044. One solution proposed in the bugreport is to add java.net.preferIPv4Stack=true but it does not work in SC. Setting bindv6only to 0 (echo 0 > /proc/sys/net/ipv6/bindv6only or set 0 in /etc/sysctl.d/bindv6only.conf file) solve the problem.
Maybe it affects other GNU/Linux distributions. But with this workaround all works fine.