I reading some articles about this , but i dont know can i use Asterisk , FreePBX for Video Conferenceing like 3cx .
If you are using Asterisk 1.4 or 1.6 then you can use most any video phone or softphone with video capability.
In sip_general_additional.conf you want to make sure you have the following info:
Hello again ,
When you enter sip_general_additional.conf headers, there’s a notice stating that you shouldn’t modify it. And http://www.freepbx.org/configuration_files states that the preferred file to make [general] modifications on sip.con is sip_general_custom.conf. Thus, I’m applying suggested modifications on this file.