Request timeout-408 error

hi there
i am using asterisk with freepbx for sip and also using IM on that
its on daily basis that user A sends msg to user B and when user B replied that is in other country it got the error “request timeout-408” once and then he send as many msgs he got the same error but when he close the IM chat window and start a new sesssion (user B) then he able to send the msg to user A and the conversation goes perfect
but the thing is this why he got the error
anyone please help it happens daily with all the users and then they need to close the session and starts a new session and then things goes well…

i am attaching a log file that might be helpfull to see