This is usually a NAT problem
If you look at the link printed in your console output...
https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions
...it will give you a much better explanation than I can. Usually, this is caused by something you aren't doing correctly; either in Asterisk or in your firewall. If you are running SAIL server-only then check that you have included the correct external IP address in globals. Also check that your firewall is open to 5060 and 10000-20000 and that they are forwarded to SAIL.
Kind Regards
S