Hello Gracedman,
Thanks for your detail reply. I have followed above procedure and i am happy to see at least something is happening

. Now from zimbra asterisk zimlet when i dail any extension it gives me message first connecting to 4568 then another pop up message appears saying call successfully established from to 4568. but after this nothing happens if i check my asterisk log i am getting this message given below.
== Using SIP RTP CoS mark 5
If i check my zimbra log then getting this message.
/opt/zimbra/log/mailbox.log
2009-10-29 09:15:24,251 INFO [btpool0-8] [name=sikander@mail.com;mid=3; ip=192.168.50.180;ua=ZimbraWebClient - FF3.0 (Win)/5.0.18_GA_3011.RHEL5;] soap - NoOpRequest
If i dial any extension using my x-lite soft phone it is completely working OK.
Is there any other place where i can check the logs to resolve this problem. Am i going to the right track?
Thanks
Sikander