It sounds like a bug, or possibly that BVI isn't supposed to work at all. Much of the bridging capability was removed from the TA900 firmware a long time ago as we had several units break during an upgrade.
Probably best to open a support case. You might be better off front-ending it with a small switch and not using IRB. The Netvanta 1531 is a nice switch and relatively inexpensive.
See this post regarding the same question Re: media-gateway ip primary on interface bvi 1
Unfortunately, I cannot speak to how the feature is functioning. However, I can confirm that, at the time of this post, the fact that you are able to set this option in the GUI is an errata and that "media-gateway ip primary" is not a supported setting on the BVI interface.
See also this thread on why the BVI is a bad idea for VoIP since you can't apply any QoS to it Re: 3448 ARP issue with Cox
Layer 3 QoS maps are assigned to the layer 3 interface of an AOS device (an interface that contains an IP address). Once you enable bridging and add the ethernet interface to the bridge-group, this interface no longer has layer 3 functionality. All interfaces that are included in the bridge-group are unable to do QoS.