I've been playing around the the GrandCentral mobile interface (
www.grandcentral.com/mobile) Two observations:
1. Add some dummy contacts in your contact list. It seems that you can edit a contact on the mobile page but not add one.
2. It looks like this may be a good way to make cheap calls when traveling. If public pay phones in airports will still ring for incoming calls, you can trigger a call between any temporary location and anyone in your contact list. So, while I have not actually tried this with a pay phone, you might be able to set a temporary call-to number, trigger a call to a contact and then unset the temporary number.