This is working as designed (WAD).
The PIX Firewall does not support Cisco VT Advantage (VTA) at this point.
The particular Skinny video protocol version that Cisco CallManager 4.x uses is currently not supported with the PIX Operating System (OS). Therefore, the video phones do not function through a PIX box.
Version 7x of PIX OS will have the fixes.
For related information, refer to:
CallManager to use VT Advantage Configuration Example