Running a TrixBox behind a firewall can cause one-way audio issues. On a Trixbox, you will need to specify the external IP as well as the internal network

in sip.conf.

ie:

[general]
externhost = asterisk.company.com
externip=123.123.123.123 ; the ip address your ISP gives you
localnet=192.168.1.0/255.255.255.0
nat=yes
port = 5060 ; Port to bind to (SIP is 5060)
bindaddr = 0.0.0.0 ; Address to bind to (all addresses on machine)
disallow=all ; Modify these to meet your codec needs
allow=ulaw
allow=alaw
allow=g729
allow=gsm