VoIP Calls Lose Audio after Call Transfer

[Guest]

I'm having a very odd problem using pfSense 1.2.2.  I have 4 Linksys SPA-942 VoIP phones connecting to a Trixbox PBX with a public IP address (at another location).  I have set up pfSense with AON and left the default rule as is.  Everything works perfectly except for 1 thing!

When I transfer a call from one extension to another, they lose audio in both directions.

Here is the diagram how how things are setup:
Trixbox pbx – internet -- pfsense -- lan switch -- voip phones

I'm new to pfSense but have read nearly all of the NAT issues with VoIP and none of them have solved my issue.  Any help would be GREATLY appreciated!

Thanks in advance,

Michael