Interesting VoIP Forum posting. Any takers to answer his question?
I just added AT&T Callvantage to my home network. I had upgraded to
Comcast's Home Worker/Teleworker option. Realized speeeds of 6 Mbps down and 768Kbps up. Once I put that MTA inline, it drops to 1.5-1.8 Mbps down and about 500 Kbps up. 
Any ideas? YES I did remove the device and was able to regain the speed so there appears to be a throttle in there. It is a Centillium device with a Motorola modem ahead of it and a D-Link wireless router behind it..
Any help is appreciated..
