Environment :

• CentOS 6.2 64bit
• Asterisk 1.8.20.1
• Asterisk machine (VM) behind NAT (Port Forwading)
• Public IP : 114.13.10.85
• Asterisk local IP : 10.10.10.1

CLI Log :

[Feb 28 09:28:22] WARNING[13394]: chan_sip.c:3974 retrans_pkt: Retransmission timeout reached on transmission ce69b44af71ba023NjgzZjFiNTY1MDc5MGJhNzA0ZjBjN2RiZjVlMzUzOTc. for seqno 1 (Critical Response) -- See https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions
Packet timed out after 16576ms with no response
[Feb 28 09:28:35] WARNING[13394]: chan_sip.c:3974 retrans_pkt: Retransmission timeout reached on transmission aa68952b465def6eNjgzZjFiNTY1MDc5MGJhNzA0ZjBjN2RiZjVlMzUzOTc. for seqno 1 (Critical Response) -- See https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions
Packet timed out after 16575ms with no response

Step to fix problem :

• Change sip.conf (/etc/asterisk/) configuration, add this line :

localnet = 10.10.10.0/255.255.255.0
externaddr = 114.13.10.85

• Reload Asterisk configuration

[root@ivr ~]# asterisk -vvvvvr
ivr*CLI> reload