You're right about the URLs being pretty clear. If you're logged in/it remembers you, and you go to http://www.grandcentral.com/mobile/c...tno=5555555555 where the 5s are the number you're trying to call, it'll take you to a page that says your browser didn't send a valid referring url but with a link ("click here to try again") that will dial the number.

The full url when it actually calls has a string after the numbers that, presumably, is tied to your session.

Even a quick way to go to that page with a number of your choice would be more convenient than always having to dial from your GC contacts, I think.