I suspect that most people smart enough to read slashdot already know this, but: The IRS will never make first contact via phone. They will make first contact by mail. That is paper mail. They will not use e-mail much less facebook, IM, or text.
Unless you are already actively working with (or against) someone in the IRS regarding your case and are expecting contact through other means, you can be confident that any phone calls you receive are fraudulent.
Also, and even more blatantly, the IRS will not take payment in the form of gift cards.