Are you unphased or unfazed?

Which is correct?
The user-training for the new business intelligence software was extensive, in the hopes that employees would be __________ by the change.
a. unphased
b. unfazed

Answer: b.

Explanation:
A phase is a stage; the word is usually a noun. People may go through phases but we don’t say they’re phased when they do so.

Faze, on the other hand, only means to disrupt someone’s composure. It comes from an Old English word, fésian, meaning to frighten or to startle into flight.

Interestingly, we only seem to use faze in the negative sense. Someone is not fazed by something or was unfazed — you don’t hear about people being fazed.

I’ve also seen faced used to mean fazed. But as the Urban Dictionary points out, to say someone is faced, without specifying that they’re facing something in particular, is really just shorthand for saying they’re sh*t-faced, meaning extremely intoxicated. So if you said someone was unfaced during the executive meeting, that would be faint praise.

The Eggcorn Database covers faze/phase and Mark Liberman also has an entertaining post about it.