hi,
I use asterisk based telephony system. but calls drop after a while and sometimes we can not send a fax. some of our user can not calling eachother properly.

can you give me exact solution?

asterisk runs on centos 6.x
we use esxi too.
we use ip pbx, ip telephones too
can someone tell me whole network , nat and firewall rules or some speciphic configuration settings.
if we active t38 for faxes , what should be the carries configurations on network or firewall or nat?
our cables are fiberoptic. is it possible that any man in the middle or replication our network settings by remotely?

any idea?

Welcome to LinuxQuestions.org. Your opening post reads like you know very little about Asterisk, and you have wide-ranging questions, that I doubt can be answered comprehensively with the vague statements/descritions you've supplied. Are you responsible for setting this up and maintaining it? If so, I think you should start from the beginning, reading documentation and wiki pages. The Asterisk site contains a lot of useful resources....

https://www.asterisk.org/get-started
https://wiki.archlinux.org/index.php/Asterisk