Phone screenings do not require signing an NDA (just a "we explicitly ask you not to share the questions you're asked"), but if they bring you on site you will be asked to sign one.

Except when not. When I was interviewing they didn't make me sign one. They rejected me for the original position, but later occasionally called me with some alternate options. These calls usually went like:

- We would be interested in having you at/as ... but before the specifics, you signed an NDA, right?

- No, I have not.

- Oh, then good luck, bye.

They're doing it ever so slightly wrong if they just take your word for it like that!

When I interviewed they did not ask for an NDA during the phone interviews, but they did have me sign one before the on-site interview.

For what it's worth, I had an overall positive experience interviewing with Google.

The phone interviews are not but the in person ones are. I'm sure it has to do with the fact that you are onsite and may oversee confidential information but they explicitly stated to me that it covered the interview questions.