I didn't do this by calling, but when I checked in online -- no human contact involved. I believe if you were not calling within 24 hours of your original flight, you wouldn't be offered the standby. If you were within 24 hours of your original flight, perhaps you should try it on the online checkin or just call back.
Yes, but according to the CO agent, your original booking class needs to be available to switch flights that are not on the same day as your original flight. If not, you need to upgrade the fare in addition to the 25/50 USD fee