Part of my standard phone screen approach is to open up an etherpad or other collaborative coding tool and ask the candidate to perform a Fizzbuzz variant (variant enough that memorizing Fizzbuzz wouldn't save you). This is entirely intended for failing people who can't code at all.

Yes, I've had people fail this. Yes, I gave them an extra day post-phone-screen to complete it. No, they did not complete it.

While I loathe the alpha male dance of software engineering interviews, I fully support the Fizzbuzz test, having encountered those that fail it.