Hi,
We ran into an issue a week ago when this warning message came across our system multiple times. Once the system started flooding this message we noticed all inbound calls from our SIP Trunks could be answered but no audio on either end could be heard. Also, once you hung up a call that was inbound, the caller would appear to be redialing the phone multiple times like 5 or 6 times before it would stop. I know for a fact that they really weren't re-dialing because I tested with my cell phone as well and once I hung up on my Polycom, sure it enough it appeared that my cell phone was calling my Polycom on 4 different lines! Here is the warning message that we came across:
[2012-10-02 21:56:24] WARNING[20279] chan_sip.c: Retransmission timeout reached on transmission 14a2afcb4e8740f0592719864359851b@PUBLICIPOFOURPBX:5060 for seqno 102 (Critical Request) -- See https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions
Our PBX isn't NAT'ed as it sits on a Public IP. The solution that fixed this issue was replacing the primary PBX with our backup and instantly the warning messages went away and all went back to normal. We completely restored our production system and is now back in place. Any thoughts or advice as to what to do to help prevent this from happening again?