You're looking for the Advanced Voice Busyout feature, specifically local voice busyout:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t3/dt_avbo.htm#47397
Create a busyout voice-class that monitors your Fast Ethernet interface, and then apply that to the voice-port you would like to busyout.
If you configure your MGCP gateway with ccm-manager fallback-mgcp, this feature should be able to take effect once the router shifts in to H.323 mode. It may have no effect while in MGCP mode. However, when you disconnect the router's Ethernet interface, it will lose contact with CallManager according to your drawing, and thus fall back to H.323 where it will definitely take effect. I haven't tested this, but it should work. If it doesn't work at all, and it's an important requirement for you, you'll need to reconfigure your router as an H.323 gateway instead of MGCP.
