New error appeared and the calls started dropping!

hi ,
today im facing a new issue , which is dropping calls during calls not on initiating of calls .
actually i didn’t modify anything from my server config .

i noted the new error appeared in the file
/var/log/asterisk/full

[color=#FF0000][Oct 24 14:57:13] WARNING[2074] chan_sip.c: Re-invite to non-existing call leg on other UA. SIP dialog '337f2042413a38tyuf34abc9545d2568c1@x.x.x.x:5060’. Giving up.[/color]

if a dropping in calls occurred , then this error is shown in the log .

i have the system specifications :
[i]Kernel
Linux(i386)-2.6.18-238.12.1.el5

Elastix
elastix-2.2.0-14
elastix-firstboot-2.2.0-5
elastix-system-2.2.0-14
elastix-email_admin-2.2.0-9
elastix-vtigercrm-5.1.0-8
elastix-extras-2.0.4-4
elastix-asterisk-sounds-1.2.3-1
elastix-my_extension-2.2.0-5
elastix-agenda-2.2.0-5
elastix-a2billing-1.8.1-16
elastix-addons-2.2.0-4
elastix-im-2.0.4-2
elastix-pbx-2.2.0-14
elastix-security-2.2.0-7
elastix-reports-2.2.0-6
elastix-fax-2.2.0-4

RounCubeMail
RoundCubeMail-0.3.1-10

Mail
postfix-2.3.3-2.3.el5_6
cyrus-imapd-2.3.7-12.el5_7.1

IM
openfire-3.5.1-2

FreePBX
freePBX-2.8.1-7

Asterisk
asterisk-1.8.7.0-0
asterisk-perl-0.10-2
asterisk-addons-1.8.7.0-0

FAX
hylafax-4.3.10-2rhel5
iaxmodem-1.2.0-1.1

DRIVERS
dahdi-2.4.1.2-5
rhino-0.99.4-2.rc1
wanpipe-util-3.5.23-1
[/i]

please advice about this issue

regards