Hey Joe -
Just to clarify not all systems are set up like that. Our system uses separate CMTS just for VoIP. It's not tied into the public internet. If the VoIP CMTS sees a cable modem it kicks it off and the modem switches over to out HSD CMTS. The same goes for the EMTA on an HSD CMTS.
I believe Comcast is using a single CMTS for both VoIP and HSD like you say.
Scott