Audio problem & calls drop after a while & fax send problem

I have asterisk based ( running on centos 6.x [ esxi ] ) telephony system. there is IP PBX, mediant too.

we have problems : we can not send fax , sometimes system does not work properly,
generally calls drop after a while ( cli gives retransmission warning or provider warning ),
on the other hand system sometimes work very slowly, our smtp mail does not work properly and local ip telephones routes sometimes mixture end points , namely it does not take their extensions, main pbx sometimes routes arbitrarily or some telephones.

I will be appriciated clues,hints, solutions about firewall,nat, sip, stun, mediant,configuration solutions too.

best regards,