I had have the same problem with Urmet's PABX. I resolved the problem using the module VIC-2DID that act as FXS, whith the feature that is possible change the output voltage. When you install this FXS you must insert those commands in voice-port conf...
If you use the new version 6.3.1 the PIX understand the 802.1q encapsulation:http://www.cisco.com/en/US/products/sw/secursw/ps2120/prod_release_note09186a008015e582.htmlRegards, Gilberto