issable pbx 5 Mute Issue on call

i have issable pbx 5 system and i installed it on vps
i connect vpn ssl vpn on vps to get access to trunk
so
this is my trunk config

type=peer
host=192.168.4.10
port=5060
username=8015
secret=********
fromuser=8015
fromdomain=192.168.4.10
disallow=all
allow=ulaw&alaw&g729
insecure=invite,port
qualify=yes
nat=no

The exact problem is that after answering the call for 30 seconds or more, the sound is muted, not for all calls, but for some calls.