I can not send fax and calls drop immediately on asterisk based system

firstly our system sometimes run very slowly, can a reason com from isp or something used ram more than usual?

secondly we have a telephony system runs on centos ( esxi ) asterisk ip pbx system.
sometimes we face audio problem, calls drop after a while,sometimes local ip telephones redirect wrong end points arbitrarily callings. sometimes our general mail system does not work properly. we can not send or receive mails used on this system.

any advice? and I will be appriciated firewall , nat and routing solutions too.

b. regards,