I completely agree with you. Once you go past the 15 minute mark, the whiteboard is no longer a good way to do programming aptitude testing. It's a good smoke test, but doesn't really work beyond that.
Of the three examples you mention, I think only the phone number alphabetic representation is a reasonable one to do on a whiteboard.
All the 5 to 10 whiteboard interviews I've had over the years have been smoke tests. This is in Europe, and I got the impression from this thread that it's a common smoke test in Europe and Americans tend to have longer whiteboard interviews.
Of the three examples you mention, I think only the phone number alphabetic representation is a reasonable one to do on a whiteboard.
All the 5 to 10 whiteboard interviews I've had over the years have been smoke tests. This is in Europe, and I got the impression from this thread that it's a common smoke test in Europe and Americans tend to have longer whiteboard interviews.