I use a lot of PBX in a flash distros and what I have found is if you can setup a rule in NAT for 10000 - 20000 UDP inbound to the IP of your Trixbox and don't setup an inbound NAT rule for for 5060. Uninstall SIP Prox and restart your pfsense, make sure your trixbox rtp.conf file is setup to use only ports 10000 - 20000.
I have four sites working like this each with pfSense 2.01 three different VoIP providers and no issues.
I would also setup a NAT outbound rule for your Trixbox to use a static Port as per the below example.
WAN 192.168.0.8/31 * * * * * YES VoIP Server