The gateway address in the Apex is incorrect. It should be 10.0.1.1. Change that, reboot the Apex, and try again.
The Vonage is probably a router. What is the Internet address of the Extreme? I'm guessing that it is a 192.168.15.x address.
If changing the Apex gateway doesn't help, take the Vonage device out of the picture for a few minutes, connecting the Airport directly to the cable modem. See if external access works.
Although Vonage says their device must be in front of your router, they usually work ok when connected behind the router. You might need to do that.