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..
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likely to be the dlink box unable to keep up with the traffic
>>likely to be the dlink box unable to keep up with the traffic
He did mention the speed was fine with just the D-Link and without CallVantage's MTA.
I think he is trying to front-end his firewall with the Centillium MTA so he gets the QoS advantage "advertised" by AT&T if you front end your firewall with the MTA.
If that's the case, maybe the Centillium MTA is hard coded in the firmware with a cap on the bandwidth? Might be a way to logon to the box via a web browser and check - assuming the device isn't password protected.
He might be better off just putting the MTA behind his D-Link firewall if his D-link firewall can handle higher speeds. But he'll lose the QoS voice packet prioritization capability.
I'll contact my sources at Centillium and see what's up.