I know that the Gateway module on the IP/VC 3540 has the option to roll over to ISDN for when a determined number of channels has been used, this is for voice.Can it roll over to ISDN for Video over IP when a determined bandwidth has been reached?

We're implementing the Cisco IP/VC 3540 and are going to use a 3640 router for the gatekeeper. Does the gatekeeper require it's own IP address or is it assigned the IP for one of the interfaces on the router?